The great British v Exotic Diving Debate. At first glance, it is very simple. Ask any divers who learned to dive in British waters: "Where's the best diving in the world?", and they will tell you - Britain.
But tell that to any divers who learned to dive abroad, and they will think you're bloody mad.
This is because the most important dive you make - the one where your adrenalin levels are off the scale, when your brain reviews your gauges 60 times per second, when the sheer force of your heartbeat breaks the zip on your stiff new wetsuit - this dive is your first dive.
And because you are functioning at twice the humanly feasible metabolic rate, the impression of that first descent is etched twice as deeply on your memory as any other.
Makes sense, doesn't it? Initially, yes. But how is it that I learned to dive in the Red Sea, and yet would rather dive on the Rondo in Loch Sunart than the Rhone in the British Virgin Islands?
This is not just a macho pose, nor the sour-grapes reaction of some sad pillock who only dives off the beach at Marsk because he has never had the wit to get on a plane.
I've been about a bit, me. I love the Caribbean. I love it because you can get a tan at 20m; because some latter-day Jeeves is always on hand to lick the plankton off your weightbelt and dab the excess condensation from your Budweiser; because you doze away your tween-dive intervals bobbing under a cloudless sky of palm-fringed, coral beaches; and because - let's be frank, for Chrissake - if I'm writing about it someone else picks up the tab.
But as for the diving, it is like ticking off entries in I Spy Underwater.
If you boast a DPhil in marine biology and have an unhealthy interest in the mating dance of the Arrowhead Crab, then the very monotony of coral diving is what you thrive upon. "Look! Did you see that? He wiggled his left pincer a good two millimetres to the left! Lord, he's never done that before!"
But if, like me, you are the average, interested-but-largely-ignorant bloke on the Brixham omnibus type, and you see anything on Dive 3 that you didn't see on Dive 1, then you are either dead lucky or supernaturally observant, or under the influence of a hallucinogenic drug.
The fact is, with very little inducement beyond the diver's gasp of appreciation, the inhabitants of coral waters will form an orderly queue and file slowly past him, presenting their best profile.
For a bit of stale pitta bread, they will sing Verdi's Requiem.
Your average tropical marine animal is far more organised than its diving-instructor counterpart. For instance, the sharks work a shift system: "Right! Callin' all 'ammer- 'eads! Get your 'ungry look on, the 9.30 dive's due in two minutes.
"Oi! Where are you goin', matey? Have you suddenly sprouted an 'ammer-shaped protuberance in the cranial area, perchance? No, I think not! Accordin' to my notes, you are a White-Tipped Reef Shark and as such, not due on till 15.45. So get back in yer cave, and look glazed an' soporific!"
Some species prefer to work nights: "The getting-up's a bit of a struggle at first," the Caribbean Lobster will admit if pushed, "but the traffic's lighter and you meet a nicer class of crustacean."
My point is this: that while you can set your watch by a turtle in the Bahamas, and while any decent skipper in the Maldives can drop you where there are more mantas than there are crabs on Christmas Island, in UK waters you are bloody lucky if you see anything at all.
This is the unique quality of British diving that makes it the most challenging and rewarding in the world.
"Did you see it?"
"Did I see what?"
"Oh, for God's sake! Don't tell me you didn't see it!"
"You mean the kelp? That brown thing I thought was an eel? That was kelp, that was."
"No, no, not the kelp! Sometimes I despair, I really do. I take you to the best dive site in England, I get us dropped bang on the bleeding marks, the viz is perfect, three feet at least, and you miss it!"
"What did I miss? Tell me!"
"The periwinkle, you blind bastard! I've never seen anything like it! It was gigantic - the size of... a pound coin, at least! And you missed it!"
British diving comes in for a lot of bashing from people who have nev done it. Take the visibility thing. The fair-weather lot look genuinely surprised when you tell them you have dived on a wreck in the Channel. "What, can you see anything down there?"
And you sigh and run through The Explanation once more: how the water is not polluted as such, how poor visibility is caused by sediment in solution, or by a plankton bloom, how sometimes the viz is spectacularly good - ie, when the sea has been flat-calm for at least four days, the dive site is off a rocky shore, you happen to hit a lull in the plankton reproductive cycle, and the sun is high enough to penetrate the surface.
Such conditions, you point out, can occur as frequently as once or twice every ten years.
SO what makes us do it? British diving is usually inconvenient. If you have a boat, you have to tow, launch and load it. If you don't, you are limited to one of the three half-decent shore dives in 6000 miles of coastline.
For most of the year, it is uncomfortably cold. Last August off Cornwall (the so-called English Riviera) I was reduced to ladling the water from the engine outlet down the back of my wetsuit.
The marine life, as in the tropics, is a fair reflection of that onshore. Or to put it another way, compare the people of Antigua, laughing and dancing the night away beneath the palm trees to the rhythms of the steel band, to Mr Smedley, the retired ledger clerk in the bungalow near Kingsbridge.
This tells you more about the fauna of the North Atlantic than any marine guide.
So why we love British diving is a bit of a mystery. Here's my theory, for what it's worth. It is our early fascination with the sea that drives us to dive. And for most of us, that sea was the North, the Channel or the Atlantic.
Looking through my notebooks, I found this entry:
"I was born in the next bed to William David Callender Dawson, my best friend. Our folks owned holiday homes at Runswick Bay near Whitby. (Bram Stoker has Dracula flit over its southern headland, a place of truly Gothic desolation.)
"We both possessed boats and we were rarely out of them. Mine was a plywood pram called the Lusitania.
"I used to think that Runswick Bay was terribly deep. But in the single most exciting adventure of my diving career, I explored its plunging abysses with a mask and snorkel. I'm afraid I must report that the Marianas Trench of my childhood is nowhere deeper than 10m."
Above all, diving is a romantic experience, and what is romantic when you are nine is romantic forever. I love the innate hostility of British water. Nowhere else do you enjoy the same sense of being where you are not supposed to be.
Diving in the tropics is warm and bright and friendly, like swimming in a rich man's aquarium. Diving in Britain is gloomy and mysterious: the landscape unfolds reluctantly, guards its secrets jealously.
In the tropics, a wreck is a sunny sort of place, an artificial reef, a holiday camp for vivid fish. In the Sound of Mull, a wreck is a grim testament to a tragedy, a grave and a warning.
Two thousand years ago, the Romans renounced the warmth and effortless fertility of Italy and, haunted by images of wild moors, mists and marshes, embraced Britannia's endless winter.
Perhaps that perverse asceticism has filtered down the generations, so that we prefer our pleasures cold and hard.
In fact it must so, otherwise we would all have buggered off years ago to live in St Tropez.
That is my theory, anyway. Take it or leave it. But don't take it too seriously. After all, I'm off to Cuba next week.
Three Great British Divesites in this month's Diver: