The current was unusually strong that day. When currents are strong it is normal to descend quickly to get beneath them. And so we did. Divers descending in a group are a bit like parachutists. They drop in an array and regroup on the floor. The danger of a strong current is that it disperses the group and one of the foundational rules of diving is that you never dive alone. For safety reasons, among other considerations, it is a group activity. That day I descended a bit too fast.

I was conscious that something was wrong when my mask wouldn't clear. Nothing especially unusual in that and there are procedures to correct the problem. I tried the usual tricks and the problem remained. I did them again. By this time I'd more or less reached the bottom. I removed my mask to have a look. At that point my eyesight vanished in a blur of foamy-green nothingness. I fiddled around with the mask and placed it back on my face. The problem was worse. The mask flooded with water. I played around with it again and felt something give. The glass in the mask popped out and spun downwards past the momentarily curious fish.

It is hard to communicate what it feels like to be 20m beneath the ocean's surface and to be blind and alone. Your visual field is like an ill-tuned television into which lurch indistinct shapes and colours, none very clearly human. I was also irrationally concerned that among our destinations that day were, first, a large trench dropping to depths way beyond the capacities of either my skills or my tank, and, second, a place where sharks were known to take their rest. My anxieties were foolish because we were then nowhere near either place. But one does not want to wander blind into a shark's bedroom.

I understand that in a condition known as recovered sight - when a blind person regains vision after surgery - it is not, as one might imagine, like turning on a light in a dark room. What the person sees is a blizzard of meaningless signals: colours, shapes, forms, movements. Thus, functionally blind, was I on a reef below the Maldives.

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Normally, I find descents serene and think of them as one of the best parts of the dive. You go down slowly, carefully equalising the pressure difference between your ears and the ever-increasing squeeze of the water. Oddly it feels like floating when in fact you are sinking. This is one of the curious, and slightly oxymoronic, pleasures of the underwater experience: a sort of between-states feeling, of inhabiting neither the world of solids nor of gasses.

Following the contours of the huge rock walls of the Mediterranean, which descend vertically beyond sight beneath you, feels like flying. You can spread your arms wide, mimicking skydiving, as suspended there you gaze enthralled at slow, ruminative groupers, or fast, sleek tuna rising from the deep. This feeling is intriguing to me because I hate flying and would run screaming from the suggestion of skydiving. But in the liquid world you enjoy the support of matter and the illusion of air.

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Back in the Maldives, being on the sea floor and unable to see can be comical. When you dive, you dive in pairs, with a "buddy". And, because "the vis" may be poor, you take careful note of important distinguishing marks among your companions, like their brightly coloured fins. (Bad fashion sense isn't the only reason divers dress as they do.)

That day my buddy was wearing bright yellow fins. As I peered around, I saw a bright yellow shape of seemingly the right sort at about head-height and an arm's length away. I grabbed at it in relief, and grabbed at nothing. The yellow shape hovered, descending slightly. I cursed my buddy's inattentiveness and grabbed again: no doubt I made annoyed, sub-articulate noises like a man with a gag (which is, in fact, what the regulator is). Still the shape fluttered and ignored me. It took me several more lunges and, in my faintly panicked state, much longer than it should have done to figure out that the yellow shape at which I was clownishly lunging was, in fact, a fish.

What is so enticing about the underwater world? Physical grace: I imagine it's like dancing for those who can, instead of being, like me, a person shifting gormlessly from foot to foot. But with 15m of water over my head, now we're talking. You can shimmer, glide and hover with balance while the bubbles rise in slow spirals around you. Another of the paradoxes of diving is the relationship between mass and emptiness. There's a calming insubstantiality about being at depth, an experience of both heavy materiality (have you ever tried to pick up a scuba tank?) and a very bearable lightness of being. Behind you the awkward fumbling characteristic of shoreline entries when, fully kitted-up in heavy surf, you blunder towards a thin crack in a reef's edge, banging your knees and elbows on hard, resistant things. Before you is tranquil liberty, as you push through the same water whose eruptive surface moments ago threatened stability, injury and temper. Now it supports your every movement. You are poised. It is the only time I ever feel physically graceful.

Then there's the light. Though some divers have a fetish for depth, and serious depth does have its attractions, the best stuff for a plodder like me is usually in the first 20m. I'd rather warm water, a wide variety of coral and strong sun above. The most abundant, colourful and zany, if not the most dramatic forms of aquatic life, live in the top 15m. Watching the play of strong sunlight over the breathtaking forms of coral heads or waving vegetation, with fish going through them at all speeds imaginable, and possessing all the colours conceivable, is bliss.

Drift diving - when the boat drops you in one place in a strong current and collects you further downstream - is for this reason an even purer bliss. You just float through the world. Natural history TV portrays the dark and weird deeps, the coilings and aggressions of the grotesque and unpleasant. But a coral reef is a doll's house of miniature delights that you observe but never touch. This is a diver's rule: if it looks especially beautiful, or especially ugly, leave it alone. (It's a good rule in many contexts.) An ideal dive should leave no trace of ever having happened.

Back on the sea floor, it was time to think carefully. Of course there was no real danger in my predicament, other than that produced by being suddenly severed from one's most important mode of interpreting the world and starting to panic. My yellow fish and I, now quite companionable, puzzled together. I needed either to find the group, where someone might have a spare mask, or surface slowly and rejoin the boat. I tried to calculate how far I might have drifted in the current relative to others. That was hopeless: too many variables; not enough orientation. So then the question is, like the climber's dilemma when lost in cloud: should I stay or should I go? - as The Clash used to sing.

Underwater, the advice is to stay for a short time when separated from one's buddy, before making an orderly ascent with proper precautions to decompress. Which is good advice, but harder to operate with composure when the water is streaming over your eyes and you suddenly wonder where the boat might have gone.

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Among the pleasures of diving is surprise. Someone once told me about a dull dive she'd done in the Red Sea in poor visibility, when suddenly out of nowhere - think of things looming out of thick fogs - a huge whale shark (they are harmless) materialised beside her. They swam along together for a while before off it went, back into the deep and the dark. We went back the next day to the same spot but of course without the same result. It is like winning the lottery one week and planning to do so again the next. Divers have these encounters, brief imaginary companionships with a large animal in its world, sharing a small amount of time, momentous for you, insignificant for it.

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With some, like giant turtles, this can be protracted and you can follow them around as they gnaw at the coral, returning briefly to the surface to breathe until they finally go off over the edge of the shelf and into the darkness. Manta rays, too, perhaps the most graceful of large creatures, turn in methodical sweeping circles, hoovering up plankton. You tread water; they come towards you within a yard or so, mouths enormously agape. Then they dip below, turning with sweet grace to repeat their rhythmic beat back and forth. You can see the intricate, soft-seeming pelt of their hides, sometimes scarred savagely by propeller blades.

Some creatures interact. Turtles can do, or seem to. Young stingrays can be very playful, allowing you to swim after them, then stopping, looking back, going a little further, towing you by fascination. I've known them to swim between wide-apart legs in the shallows, as small children do in a pool.

Sharks, on the other hand, are shy and tend to flee. One's first encounter with a shark is always memorable; they are so burdened with mythology. In my case, years ago in Malaysia, I had the habit of swimming around a headland reef each morning with a mask and snorkel, when substantial grey creatures flashed across my peripheral vision.

"Those look like sharks," I thought, scepticism keeping me calm. At the dive centre I reported what I thought I'd seen. "Oh, yes," they said, "There are lots." They were juvenile reef sharks, two or three of them, probably 4ft-5ft long, but travelling at such unbelievable speed it was difficult to get a clear sighting. Further out from the reef they grow larger, 12ft or more, but for the juveniles the reef is such an ample source of food they give no thought to large humans as prey, and their shyness takes them away as fast as possible. But their speed haunts you nonetheless.

And then there are the small things. Coral reefs are such extremely beautiful places, and theatres of such surprise, that you would gasp continually were it not for the respirator between your teeth. The most beautiful things are often the tiniest, and it is the quality of rapt, enquiring attentiveness to what might come your way that is intoxicating. It is not, I think, too different in this respect from research: except you don't have to bother writing it down, or up. The enchantment is complete in the moment. The changeability of this world is as absorbing as its delicacy.

The same place at night is transformed, refreshed, as new creatures inhabit the same space in a different ecology. There is a well-known but still enthralling conjuring trick on a night dive. Sit still on the sea floor. Turn out the torches and you have darkness beyond anything you've ever not-seen. Wait. After a few moments you can write your name in silver phosphorescence on the black, invisible water. Something from nothing: a trick of light and chemicals. Gone.

On my day of blindness, just as I had missed the group, so it had missed me. It came looking and found me easily. They were never far away. There was a pantomime of explanations by gesture, but no spare mask. My buddy and I ascended. The others went in pursuit of the slumbering sharks, which they never found.

Surfacing, there was the boat with lounging crew. On board, everyone marvelled at my glassless eyewear and there was a good deal of exclamation and gesture (the dive centre was Italian-owned): "Never known such a thing!"

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I suppose if you are going to have a freak equipment failure, this harmless oddity might as well be the topic for animated conversation. And here is another of the enriching paradoxes of diving: the contrast between the individual diver, enclosed in neoprene isolation in clinging wetsuit, festooned with equipment and cut off from language, and the sharing of this exploratory experience with the group with whom you dive. Again, it's just like research, in its way.