Kintsugi, or Kintsukuroi is the centuries-old Japanese art of repairing broken ceramics with gold, silver or platinum. The scars and seams are not hidden, but visible: a golden root system. Somehow the object becomes more valued, more artful than it was before it fell. This is what fish do: they solder their own wounds shut with a silverish joinery.
 
The first time I discovered this, I was swimming in the centre of a shoal of schooling Axillary Seabream, Pagellus acarne. Either I was herding them, or they were choreographing me. It wasn’t clear. I moved in their hundreds. Right in the centre of this thicket, was an injured fish. It was like seeing a semi-precious stone. The mending of silver caught the light - it shone more than the other fish, it looked like an abstract sculpture, with a crescent of its body missing from a large fish or seabird bite.
 
Unlike birds, fishes’ appearance don't differ so much between males and females, so schooling fish have a slight cloned affect. It looks very military, very shepherded, very surreal. The more you study them and return to the same place, the more you notice their differences. The school I was swimming with were quite young and mostly all male.
 
Gender roles are not so binary as they are in our species and many fish change sex, or are hermaphrodites. Like gulls, some fishes’ characteristics change as they age and you can ‘date’ them by these changes. For others, their size and shape indicates when they swap genders. One of the most dramatic transformations is that of Wrasse. They have a kind of light bulb moment, but the light bulb grows in the top of their head. They look partially inflated. Clownfish have a matriarchal system; when the top female dies, the most dominant male becomes a female in her place.
 
Fish are some of the trickiest things to identify; their size is distorted by the water, as is their colour depending on light, threat, depth, weather, time of day, age. When you’re learning to identify marine species and you do a search online, the images that come back are almost always of a hanging: their bodies held up, in a man’s arms, laid in ice, or on a plate. I had not anticipated this. I expected them to be alive and in water. It is as strange to me as it would be if all the images of Blackbirds, Robins and Goldfinches, were not of them in flight, or perched, but fried and bald, or swinging upside down with their beaks pierced. The words that comes up again and again, are not species, habitat, ecology; but game, restaurant, aquarium.
 
The colours of fish underwater are extraordinary, especially turquoise. Many fish have a current of electric, azure blue roped and zig-zagged around them. A new visitor to our waters is the Grey Triggerfish, Balistes capriscus. I got a close look at one earlier this summer when it tried one of my toes. It has a wonderful blue pointillist pattern.
 
Nothing is ever the same when you go underwater: one day all the crabs will be circumnavigating a buffet of rocks, the next starfish are splayed like drunk party guests; then the day after that, there is an underwater smog, so thick you can’t see anything. My most unusual experience, was the day thousands of small salp-like organisms were near the shore, floating around me like hollow planets. I am still trying to solve what they were.

I think the poets that often come to mind when I’m swimming in the sea, or studying fish are Ted Hughes, Elizabeth Bishop, Stephanie Norgate and H.D. And for abundance - for all their plurals and collective nouns, this poem, by Galway Kinnell.

There are as many blues underwater as there are greens above. You don’t notice it when you’re down there. But afterwards, when you’re looking through photos or videos, it’s the first thing you see: Ink, Bombay Sapphire, Glacial - and you a dark putto, hovering in a sky. You can feel a bit like an icon under there with the sunbeam halos, glories and water-spectres.

I've been doing a diving course over the last few weeks. Each time our instructors wanted to teach us something, they would gather us on the seafloor in a circle, or facing them, and show, or sign to us what we needed to know and do next. I’ve learned many lessons in my underwater school: to navigate by the direction of sand ripples, or the angle of the sun; that I can make myself the weight of water, and when I weigh what water weighs, I need do nothing but lay on my tummy, gravity-less, an astronaut being swashed around a little by the tides; I’ve watched how teachers' and students' breaths become alchemy: lava-lamp like, mercury streams that merge with one anothers’, and are pulled aside like a stage curtain.
 
One of the things I was most nervous about, was one of the the things I enjoyed most. For one exercise, I had to release my air and turn it into a furious torrent - a kind of upside down waterfall which I then sipped from with the corner of my mouth. In Spanish, the word for waterfall is cascada. It was this, but the wrong way round, in this jaunty world where the ceiling seems to be the floor. The thing that blew me away, was that it tasted sweet, and that I was in water, ‘sipping’ air like a kitten. After each dive my mouth would feel a little puckered - as if I’d been blowing balloons up for too long. I was silenced by the water - exhausted, and very still, very quiet. Each night, when I lay down, I had dock rock and would be swaying and lifted in a phantom choreography. On our second dive, I missed the surface - I had culture shock: a homesickness and longing for the familiar comfort of breezes, noise, voices.
 
When you dive, red is the first colour you lose. Then orange. Yellow follows. Green. Blue. And finally, violet. You can measure and know how deep you are by their loss. The depths of the sea carry the same names as dawn and dusk and night: twilight, midnight, sunlight zone. A deep-sea diver told me that one time, she was surrounded by so much blue that the only way she knew which way was up, was by watching her air bubbles. I often think of this descent as a breaking up and dismantling of the rainbow, until only a celestial black space is left - it reminds me of a poem called The Darker Sooner by Catherine Wing.

In Richard Dawkins’ book about the partnership of art and science, Unweaving the Rainbow, he opens with a reference to John Keats.  Once, when everyone had sat down for a dinner, Keats raised his glass and proposed a toast "to the confusion of Newton". When his friend (and fellow Romantic) William Wordsworth paused before drinking, and asked what he meant, Keats replied: "Because he destroyed the poetry of the rainbow by reducing it to a prism."

It has only been a day since I finished the course and already I am missing being underwater. For me, the best classrooms are not rooms and the best lessons - not just factual, but felt: what it feels like to look a sea turtle in the eye, so close you see yourself reflected in its iris - and then to have this ancient creature, (whose species is 150-million years old) swim vertically up over your body; how it feels to always have someone beside you and you both looking out for each other (most of SCUBA teaching focuses on how to save, calm and keep each other safe); what it’s like when a species of fish you haven’t seen before drips down in front of you unexpectedly; and most of all, what it is to be in a border-less space that connects every part of our planet. You feel, truly, part rather than apart, as if you could go everywhere.

One of the world’s greatest writers, by some beautiful fortune, chose the sea as her subject. In her acceptance speech, when she won the National Book Award in 1952 for The Sea Around Us, she said:

Today, I got to kneel in loosely blown sand on the oceanfloor and write. I knelt in front of a Pink Smith Damselfish: a dive slate on my wrist, a flounder beside my knee, wet book and stubby pencil in my hand. One of the things I hadn't thought about before learning to dive was holding hands. Sometimes I feel we adults can talk a little too much with an empty space between us and no connection or understanding. The dives have been mute, and yet so full of dialogue, there's been so much laughter under the water, reassurance, affirmations, touch, eye contact that felt more like conversations, physical slapstick, support, and most of all, us all holding onto each other and not letting anyone twist off course, be lifted by the current, or float up too far. It was my friend's turn to be the balloon today and I the kiteflyer, whose hand reached up and caught hers. The level of contact is the kind you give and get when nursing and caring for someone. I felt held all day: by the water and one another.

Whenever I write poetry in the sea, I think of Rachel Carson. Like Ralph Waldo Emerson, and Henry David Thoreau, she thought of the prose she wrote as poetry, and was called the scientist-poet of the sea. Because we know land better than water, she is more well respected for Silent Spring, her final book, but it is her sea trilogy that I’ve taken into my heart and think is her most evocative writing: The Sea Around Us, The Edge of the Sea and Under the Sea-Wind. She once told a friend that after her death, she wanted to be re-incarnated as a tern.

In her life, she only got to dive once. From her writing, you would think she could only exist underwater. She made the quiet sea crackle for me when I read about and listened to the seafloor through archive recordings that she'd described - which had just been captured in the 1940s. I think often of how much the oceans need her hope and sense of wonder now, her voice.

The first breath you take underwater is a miracle. Having been hiking, backpacking, travelling and moving excessively, since I left home as a teenager, I have carried many heavy things on my back, but never breath. Never my own air. You feel how much weight water holds: when you stop wading out and fall back into the waves, then go to your knees, as if asking for forgiveness or marriage and look up at the molten, rocking surface far above you.
 
Yesterday, I went to the bottom of the sea. Not for moments and glimpses. Not for as long and as far as a single breath. For almost an hour I skimmed the bottom and sank and mistakenly surfaced and sank again and swam through the traffic of fish. I felt as slow and deliberate as a paintbrush. We were children all afternoon: myself, my friend and our two dive masters. Both of them have the presence and delight of people who have found exactly what they want to do in life and found a way to do it. One instructor, Jack is an equanimous man with exceptional taste in loud shirts, who recently set up his dive college here in Tenerife. Our other trainer, Lee - is tattooed from ceiling to floor, painted with the sea, his body is a dedication to diving and marine life, with inked octopus tentacles twisting around his arm from elbow to wrist. Both are full of kindness, consideration and mirth, and have a deep respect for the sea and how its inhabitants should be treated - not as tourist attractions or circus shows to be petted or fed - but as autonomous creatures who are not there for us to witness, or interfere with, but who we can help protect. They are my kind of people: Peter Pans on this Never-Never Island.
 
This evening, I have been mostly at the surface instead - snorkeling around some of the coast, diving down momentarily and dangling upside down, bat-like to get closer looks at adult lizard fish, splayed star fish and crabs nibbling and circumnavigating a buffet of rocks. Lots of naturalists have a patch, a beat which they survey and walk throughout the years. I feel like this about my little stretch of coast - I note the seasons under water, what’s abundant when and where. Sometimes the water is almost greasy, sometimes foggy, sometimes foaming with spume - tonight, it was as if someone had just cut their lawn after a long absence: strands of sea grass clogged the surface. I swam a little further out to avoid them, to where the rocks look more like they’re covered in ash and the water gets darker, colder, deeper and my kick stronger. I was rounding one of the peninsulas when I saw something large heading toward me. My autonomic nervous system choose fright. I stopped, breath held, statue-like. It was swimming higher than the stingrays I’d seen in this area. As it got closer I saw what it was: Caretta caretta - a Loggerhead Sea Turtle, tortuga in Spanish - with its griraffe-patterned head and flippers, its slow, undulating motion, as if brushing the water away from its sides. It is the first time in my life I have seen one underwater. It is not an area where I’ve ever known divers see them. They are a vulnerable species here, endangered and threatened elsewhere. Watching it made me think of the advice shared with David Lindo in The Urban Birder, which I have followed ever since I read it: to look at everything that moves, to look at it intently - even if it seems familiar to you and to not have preconceptions of what you will and won't see in the field - simply to go out expecting to see anything.