At first light, I leave my bunk. I get myself a flask of coffee, pick up my little VHF radio, my notebook, pencil and binoculars, pull the hatch open and crawl out onto the upper deck. I stay low, train my sights on a flock of ringed plovers at the edge of the estuary. They lift and fall in a chittering mass. It is low tide. We float in barely a foot of water. The sea has left the estuary, moved off on other business, exposing shining mudflats scored with the winding rivulets my daughter calls snake trails. The smell of it: salt, diesel and gloop. Something uncovered. Something very old and rich; not quite rotten but potent. The slap of shallow tea-coloured water against the sides of my swaying boat. The clinking of jars and test-tubes on the shelves inside.

Further up the estuary, past the make-shift boatyards at Leigh-on-Sea, past the rusted hulks of abandoned barges, are the bent silhouettes of winkle-pickers, out early with their buckets, grubbing in the mud for cockles. Some of them look small enough to be children. They slowly make their way around a few old fishing boats lolling on the mud, hulls exposed, waiting for the tide to return.

Beyond the winkle-pickers, where there is still deep water, the ro-ro ferries and container tankers plough back and forth. They never stop. Sailing upriver to London, past the power stations and pylons, chimneys and cranes, all the mechanised latticework of industry. Or downriver, past wind farms and fog nets, following the water as it spans outwards into the North Sea. And beyond the ferries and tankers, in the hazy distance, stand the border control rigs. Great metal structures. Square boxes balanced on single poles. They look like old-fashioned cameras, stiff and top-heavy. A line of them, miles out to sea, guarding the mouth of the estuary.

There’s a creaking at the back of the boat, a hatch opening and a grunt of effort as Ajay heaves his backpack and sample kit out onto the roof. I remain where I am. It is important that I re-establish the order of command on this vessel. I need to claim back the separateness I allowed him to trespass over the first night he stayed.

“Morning, gorgeous,” he says.

I look over my shoulder at him, put a finger to my lips and then gesture downwards.

“Ah,” he says. “Kid’s still asleep.”

I nod, return to my binoculars.

“Such a cute kid,” he says.

“We’ll need samples taken every hour today,” I say. “Contaminants, pesticides, plastics.”

“Yes, boss,” he says. Then: “D’you ever wonder why we do this?”

“Do what?”

“Monitor it. The endless decline. You could just draw a downward line on a graph and have done with it.”

I can hear him zipping up his waterproofs, pulling on his waders. I sip my coffee, then say: “Before, when I was on my own, I believed I was collecting data for those who might find this planet in the future, so they could see what happened to us.”

“Wow. That’s cheerful. Jesus,” he says.

“Now I have Noel, I do it for her. It might be useful.”

“I guess it changes stuff. Having a kid. Always thought I might settle down. Now I don’t know.”

Ajay’s been hitching round the world for years now, a global hobo, working the luxury cruise ships in between shifts on eco-protest boats in the Arctic. His last job was on one of the huge freedom liners, monstrous things big as hospitals, endlessly ploughing their way round the deep shipping lanes of the world, providing a permanent off-shore home for tax exiles and retired billionaires.

I met him at a climate conference ten years ago, an event hosted by the activist group we both work for, where we briefly fell into bed after a night of free organic wine. Much as we did the other night. He’s acquired dreadlocks and an Australian twang since then, despite being Essex born and bred like me. He’s appealing in a curiously uninvolving way. Perhaps that’s the nature of his appeal. It is containable. Brief.

I say: “We need samphire too.”

“Can’t believe you eat that stuff.”

“It’s good for you.”

“All right then, Mum.”

There are some things I seem to need to remember repeatedly. One of them is that I do not like jokey role-based labels. Because they are never really jokey, those demeaning little jabs.

“Take the VHF,” I say, tossing it to him. “I have work to do inside. If you need me, call me through that.”

He turns the radio around in his hands. “Haven’t seen one of these for years. On the freedom liner, they used little digital ear bud things. Really cool. I should have brought one for you.”

It doesn’t do to think of the things I could have used from that ship. Ajay would never have thought to pick them up. He doesn’t live like we do. He isn’t constantly on the look-out for stuff he might be able to use, to adapt, to repurpose. He still assumes that what he needs will be provided for him.

Noel and I are largely self-sufficient. We exist off-grid. We wash with sea-water. Eat seaweed, fish, eels, cockles, mussels, birds’ eggs. Grow salad and herbs in a plastic bag on the roof. Cook on a small propane cooker. A mini wind-turbine propped on the top deck like an old-fashioned television aerial provides us with a little bit of power, but on calm days, there’s nothing. We sleep when it’s dark, moving through the day with the sun; its arc determines ours. We have buckets covertly stationed beneath the fog nets further up the coast so that when the sea mists come rolling in, and the nets are strung with glistening beads of condensation, our buckets are there to catch the drips of fresh water before they get piped off into the main water system. We collect them after dark, when the only people moving around are people like us. And, if we need them, we get tinned goods and practical items like medicines and batteries from passing trade boats.

Our boat – Mudlark – has a small engine but it’s rarely used. Mudlark is a vessel designed to float in and out with the tides. We move soundlessly backwards and forwards. We wake up where the waters choose to take us. We drift. My tai-chi instructor used to say if someone tries to attack you, you should take their strength and use it against them. He would mime a punch coming towards him in infinite slow motion, then wrap his hand around the approaching fist and move backwards with it, like a dancer. You allow the movement to move you; you do not fight against it.

Five years ago, before I had Noel, we had a huge mid-winter storm surge and Mudlark was swept miles inland, riding the flood waters. I sat in my bunk, wearing my lifejacket, sipping vodka from a hipflask and giving myself up to a greater strength, as the wind howled outside, and my flat-bottomed boat was carried in a torrent of churning water over streets and fields and car parks and former flood defences as if it were flying. We came to rest alongside what had been a motorway flyover and was now a concrete bridge arching like a rainbow over the proud new sea. I realised I was just outside the block of flats I’d lived in as a student, now derelict and half-submerged. The past is being swept away, faster and faster.

But this place is prone to flooding, always has been. Much of it is made up of what people like to call reclaimed land – as if we had owned it before. The more accurate word would be claimed. Or stolen. Depending on the sensitivity of your moral compass. It is land that once lay beneath the sea, and humans have found ingenious ways to push back the sea so they can farm on it, or build luxury homes, or new airports. But the seas are rising and the South-East of England is tilting into the ocean and the long history of the earth tells us that what was once on top will one day be on the bottom. Mountains used to be on the sea bed. The sea bed will rise up to form mountains. And if you sail just east of here, on a calm night, they say you can hear music still playing on the submerged fairground rides of an amusement park called Adventure Island, now several fathoms down.

“Did you hear my stomach then?” says Ajay. “I’m starving. I’m no good without a cooked breakfast. A shadow of myself.” He’s manoeuvring himself off the edge of the boat, I hear his waders sink into the muddy sludge of the estuary with a long ssssschlopp.

I go back down inside the boat through the hatch and pick up the radio receiver. “Ajay, are you receiving me?”

His voice comes crackling back: “Roger that, Mudlark. Receiving you loud and clear.”

I look up to see Noel sitting on her bunk, watching me through her magnifying glass, her brown eyes huge and unblinking. The first thing I saw of Noel were her eyes. Nearly two years ago now.

“Breakfast?” I say. She nods.

Ajay’s voice comes again: “The sun’s coming out. Is that an island over there? Or Kent?”

“Kent.”

“I forgot how fast the weather changes here. The mist has vanished. You can see the rigs really clearly.”

“It’ll be back sooner than you think,” I say, and glance out of the side window towards the marshy land across the water, where wisps of fog still linger like smoke.

The border control rigs were installed years ago, the idea being that they would prevent unwanted criminal or refugee craft from entering the mouth of the Thames. But all that happened was that the criminal craft got faster and harder to detect – submersible drone boats whipping past, remote-controlled by spotty kids in Shanghai – while the refugee vessels would simply moor up underneath the rig, attaching themselves to its great rusting pole, using it as shelter while they waited for help. There was meant to be a system, a process, but it became back-logged, congested, over-loaded. These were the words used. People on the news took it in turns to explain why the system wasn’t working, while the refugee craft kept arriving, increasing in number, widening out and forming what became a kind of waterborne shanty town, a jerry-rigged collar strung around the neck of the rig.

There’s a community of sorts underneath each rig now. When the mists clear, you can see them quite clearly. The roofs and chimneys. Some days, you can hear music coming across the water. Ajay worked out there in the early days, on one of the brightly-coloured charity boats, providing food and water and clothes, but he says he got sick of activists just going over there to have their pictures taken with grubby babies. Ajay always has a reason why he had to give something up and it’s never anything to do with him. He tells me the activists now spend their time arguing with each other about responsibilities and interdiction, the jurisdiction of the high seas, the invisible boundaries of the ocean.

There was a man I knew. Vaguely. Came from London but had a holiday apartment overlooking the water near Canvey Island. Kept a big luxury yacht within view of his apartment, and had a smaller speedboat, painted jet black, that he used to shuttle between the border control rigs and the mainland, late at night.

This man would say: “There’s all sorts of nonsense talked about what goes on under them rigs, but they’re just people like you and me. And I respect that. They got stuff to sell. I got stuff to buy.” And he would open a bag to show me painted bead necklaces, hand-carved wooden toys, small plastic tubs of medication, passports. “I’m not going to say where it’s from when I’m selling it. Some people are bigoted. But you want something, I’ll get it for you. You let me know, sweetheart. You let me know what you want.”

I’d encountered him once at the mouth of the estuary. I was doing tests where the river meets the sea and the waters are both things at once – neither saltwater nor fresh, but something transitional, brackish. Industrial water: the great Thames at the end of its working life, brown and grimy with accumulated labour. Limited conditions for fish fauna. Not much diversity. Just your stubborn survivors: flounder, mullet, smelt. Unlovely creatures with names like insults.

Mudlark had inadvertently bumped up alongside his luxury yacht. I’d looked outside and seen it sitting high on the water, its smooth white prow bending out above me, blotting out the sky. Its name – Melinda – in sweeping gold lettering.

He knew me from his after-hours activity. I often sit on my roof at night. I see him come and go in his fast black boat. We have traded. He has useful things. I have useful knowledge. But the grandiose way he invited me on board Melinda suggested we’d met at a cocktail party. Different boats, different lives, I suppose. Although Melinda was made to look as un-boat-like as possible. Leather sofas. Ornamental ferns. Glass cabinets. Vast touch-screen control panels. Downlighters. Uplighters. As if the boatishness of boats was something shameful, and it were better to be cruising the Thames in a replica penthouse apartment.

“All automated,” he said, gesturing airily, “I don’t have to do nothing.”

There was a woman on the sofa in the cabin surrounded by dark screens and mirrors. Red hair. Red toenails. Glanced at me as you would a delivery boy.

The man laughed and said: “What am I saying? I have to do her every bidding.”

“Oh you,” she said, as if bored of all words.

“Caroline’s the captain of my vessel if you know what I mean.”

I smiled. He was the kind of man who required his trading partners to smile. He required from me a certain level of friendliness, a certain acquiescence. He would always want me to have a drink.

“You’ll have a drink?”

A refusal would anger him because it meant I was not going along with his version of how he did business.

“I wouldn’t say no,” I said, and heard my voice echoing his – the drawn out vowels, the pub chumminess, the oi oi saveloy bonhomie. The same way I used to emphasise my accent for taxi drivers after a night out when I was a student so they knew I was a local.

He was saying: “I like a drink. I’ve worked hard, you know? I’m not happy for that money to go to the tax man. I’m not happy for him to know every little detail.” I was nodding, smiling, while my eyes behind my sunglasses swept round the un-boatish boat, seeing little of use. No ropes, no tools. His voice faded in and out, as his yacht bounced up and down on the wash from a container ship, a series of statements and justifications: “I’m a self-made man. I’m a proud man. I like my privacy. Just doing what I can to get by.”

“That’s right,” said Caroline.

We were all admiring the view and watching each other. We were all seeking to gain.

Ajay through the radio says: “You still monitoring birds? I got a few lapwing here. I got lapwing. I got shelduck. I got plenty shelduck.”

The next time I saw the man he was in the black speedboat. It was three in the morning. He’d come barrelling up the moonlit estuary then cut the engine suddenly, using Mudlark as something to hide behind. In the distance, I could hear the officious vvv-whump vvv-whump of a police boat smacking the water, heading rapidly up another channel of the Thames. He’d sat hunched in the stern of his boat till the sound faded, then stood up and nodded at me, somehow narrower and less distinct now. Featureless. Wearing a pulled-down cap he would never wear on Melinda. He opened a small cabin door, reached in, and took out something which he threw onto my upper deck. It landed with a clank.

“For your trouble,” he said, his voice a wink, an elbow in the ribs. Behind him, the cabin door he had pushed shut swung slowly open again. A small round face, two large brown eyes, a gagged mouth.

Ajay says: “More birds. Pintails. Shovellers? Maybe shovellers.”

You get used to seeing things only in terms of what you can use. What’s essential. What can be left behind. You see a furtive man standing in a shallow-sterned boat in dark water at night. You see a child of about two or three years of age looking at you. You see plastic ties at its wrists and ankles. You see the bottle he has thrown you. You pick it up.

Ajay says: “Ringed plover. Oystercatcher.”

You think that when you make monumental decisions they will always be witnessed. This is not the case. You can make decisions that change lives and nobody will ever know. There will be just a splash, then a smoothing over of the black waters, the sound of your own breath in your ears. Bodies that go into the river will surface eventually, but usually several miles downstream, often out at sea, and who can concern themselves with yet another bloated corpse on a waterway that carries hundreds of them every year, the accidents, the drunks, the tramps, the boy-racers, the jumpers, the addicts, the lonely, the lost, their pale bodies slowly cart-wheeling beneath the surface, a slow succession of circus clowns.

Ajay through the radio says: “Did you see that thing about the President?”

“I don’t watch the news.”

“What? Nothing at all?”

“I’ve had too much of it,” I say. “I don’t need anymore.”

It is pleasant inside Mudlark in the morning. The sun slants in. Noel is reading one of her books. The weightlessness of the boat, the way it is always moving, can make it feel like we live in a kind of shimmering bubble.

A crackle of static then Ajay says: “I was thinking, if we got some kind of harpoon gun, we could hook ourselves a satellite balloon.”

“No.”

“Harpoon the balloon, pull it down, adjust the settings, send it back up – and you’re back on the net. Pirate internet via satellite!”

“I don’t want the internet.”

“What? How are you going to know anything? How can you buy anything?”

“I trade.”

“I saw a guy do it in New Zealand. It’s dead easy. Man, this mud is lethal. Nearly lost a wader then.”

“No internet. No harpoon gun.”

I look out of the window, peer down at the mud. It is pale brown like a milky chocolate pudding. Beneath the sound of the passing oil tankers and the shrieking seagulls, there is a smaller, closer sound: the tinkling of tiny streams making their way through the mud. Water, like money, is always moving. Quiet and insistent and treacherous. Mudlark shifts slightly. Noel glances at me, turns a page of her book.

“I’ve only been off the net for three days and I’m already getting the sweats,” says Ajay. “What if I’ve won a prize or something?”

He chatters on, telling me about life on the freedom liners, where he worked in the Australasian Zone, pretending to be Aboriginal. One wall of the Zone was made up of a huge glass aquarium, thirty metres high, a ‘commemoration of the Barrier Reef’, where fish scooped from the ocean could look out over the ocean.

“You gotta feel for those fish,” says Ajay.

“Yeah,” I say. I am watching the water from the window. As it rises, it forms channels in the estuary and the divisions between land and water, mud and water, land and mud, become less distinct. It is all wet, all shining.

“I know you’re not into the idea right now,” says Ajay, “but if we got you on the net, I know a guy who could get you the kit. Headset, console chair. The lot.”

“There’s no room for it.”

“Make some space.”

“I’ve got rid of everything I don’t need, Ajay. I don’t carry ballast.”

I hear the clink of metal against glass as he collects water samples. Even over the old VHF radio system, I can tell that he is chewing gum as he works. A sticky viscous sound. A small part of my attention becomes completely focused on that faint noise and waits to hear it, a corset of irritation tightening around my ribs.

Ajay says: “My back’s killing me. Probably from sleeping on the floor. I’m getting old.”

Mudlark rocks and sways.

Ajay says: “Do you think you’ll always share a bed with Noel? She’s only going to get bigger.”

The static on the radio is increasing. It’s getting harder to make him out.

I think of how Noel always wraps one arm around my neck like a monkey when I read to her at bedtime. How her breathing becomes quite strained and effortful as she pushes herself towards sleep, and then how it relaxes, suddenly, as she drops off.

Noel only has a few books. They’re hard to come by these days. Most have been compressed and recycled, ending up as fire-lighters or wall cladding. The ones she has are old favourites of mine, ragged paperbacks I got from my gran. Stories of brave sailing children, of orphan girls rescued by an absent-minded professor, and a tale of an iron man, a creature who pieces himself together from scraps found on the shore.

Books were the only things I salvaged from the end of my marriage. My husband and I were packing up our belongings, preparing to go afloat and live a different kind of life in one of the offshore communities near Tilbury when he said: “You can’t take those old books. They’ll just get damp and smelly. We agreed we’d only take what was essential.” And it was surprisingly easy then, to go out to the car with the books and to sit in the driver’s seat, considering essential and non-essential items, and to quietly let the handbrake go so that the car coasted down the drive and round the corner, moving under its own momentum. To be honest, I’ve found it to be that way ever since. Leaving. Like the releasing of something I’d awkwardly wedged into place.

Ajay says: “You were right about the mist. Comes back in quickly.”

I look out of the side window. Swathes of mist are swarming back across the marshes. The tide is rushing in, slapping at the flanks of the boat. The estuary is returning to full strength. It is only here, on the water, that you really get a sense of its power. For most people, it’s simply something to look at, a flat grey super highway where tankers roll by, right to left, distant and unknowable. But on a boat in the middle of it, as it shifts and changes and lifts you away, you realise its enormity. No longer a 2D stripe of scenery but the element you exist in; a scum-flecked mass spooling away on all sides. Imagine the weight of a square metre of water – a ton or more, easily – then imagine how many times over that is replicated in this huge body of water, and then see how easily it moves, agile and fluid, its own great weight nothing at all.

Noel puts down her book. I get my binoculars and my radio, push the front hatch open and crawl up out onto the upper deck. Mudlark is moving at some speed now. She is being spun balletically by the current as we are swept downstream.

I lie flat, turning myself round so I am looking off the back of the boat, training my sights on the edge of the estuary as it recedes into the mist. I can just make out the spot where Mudlark spent the night. There are still a few brave winkle-pickers, stubbornly working as the tide comes seething in around them, and beyond them, a smaller figure, much further out.

“Mudlark?” says Ajay.

“You’re breaking up, Ajay,” I say.

“Mudlark? I can’t see you. Where are you now?”

I turn the volume off on my radio handset.

And it is beautiful, the way we are so easily carried by the water, downriver towards the ocean. It always is. We are weightless. We are free.