I'll give you money back on any dive on which you don't see a giant turtle," the dive master said on my first day on Sipadan island. He never paid me a cent. We swam among swirling schools of barracuda, through thousands of darting coral fish and above white-tip reef sharks that looked as if they were cruising around together humming the theme tune from Jaws. By the third day we weren't even bothering to point out the five-foot turtles that were a feature of every dive.

Sipadan island has only recently appeared on the dive map, but it promotes itself as the top dive spot in South East Asia. There are now at least five dive operators on the 27-acre island. Guidebooks from only five years ago write about having to camp overnight on the beach. Then there was only one local dive operator and boats had to be hired from Bajau fishermen for the 20-mile offshore trip from the dusty coastal town of Semporna in Sabah, one of the Malaysian states of North Borneo.

The island is a classic tropical castaway location - fine white sand beaches with a fringe of palm trees; inland a tangle of jungle. What makes it extraordinary is its marine design. The sea bed around the island rises in the shape of a slender rocky spine crowned by an overhanging coral reef shaped like a giant mushroom. For decades the island was simply left to Green and Hawksbill turtles which laid their eggs here. Now the underwater landscapes of reef and soft coral leave your average snorkel snob (Barrier Reef - done it; Caribbean - swum it; Red Sea - been there) humbled. Here you have a marine spectacular, with overhangs and dramatic drops teeming with wrasse, damselfish, grouper, angelfish, snappers, butterflyfish and triggerfish as well as the bigger predators.

The first school of barracuda you ever see is a fearsome experience: spiralling in mad, tight circles, four or five to a row - swimming like a silver tube. Yet it is solitary barracuda, swimming near the surface, which are the dangerous ones. They are hunting for food, these are merely socialising.

The sharks tend to rest on sandy patches at around 50 to 70 feet down. Reef white-tips are smallish, a bit over a yard, with that little shark grin: they always seem to be form sniggering groups. They are well-fed and not a threat, the dive master told us, after sharing his "I swam with a school of hammerheads" story. Twice, while diving at a depth of around 100 feet, we saw, sliding silently below us, a lone leopard shark - elegantly spotted, with a long, almost feathered tail.

After every divecame the comparisons - cross-checking with different books as to what we saw. I get easily side-tracked by laconic descriptions of the sex lives of reef fish. The majority undergo sex reversal as part of their development. Many are sequential hermaphrodites. The changes of sex can be socially controlled. If there are too many males this inhibits primary females from changing into males, but if the ratio of male to female falls below a certain threshold, the dominant females will change sex.

However, the stars among the sexually talented, the predators and the plain showy - parrot fish, clown fish, stripey lion fish - are still the turtles. They sleep on ledges on the coral wall, doze beside rocks, and pirouette away with a lazy flip of a fin from divers who are deluded into thinking they can catch up. The very best place to watch them is at a cleaning station. We saw an enormous turtle, at least 80 years old, hover suspended, allowing small fish to dart over her shell and under her belly, cleaning as they went.The next two turtles queued up patiently.

At around 60 feet underwater along the coral wall you see occasional signs marking caves. Entry is forbidden to ordinary divers. One cave is legendary as a turtle graveyard. Without coming up for air at regular intervals turtles drown. The cave is apparently lined with the shells and skulls of turtles which have swum in and been unable to find their way out. In the early days of diving on Sipadan, two divers disregarded warnings and attempted a night dive into a cave and met the same fate - hence the signs.

On a night dive, swimming by the wavering light of two torches, we were met around a sweep of the wall by 20 torches clutched by Japanese divers, most armed with enormous underwater cameras. Thankfully, though, Sipadan may be declared a marine park soon, and a limit fixed as to the number of divers allowed at any one time. So far the limits have been the accommodation on the island and the fact that only those with at least a basic qualification can dive: no training courses are run here.

The island has huge potential in terms of money-making tourism, so the time to go there is now - while the Sipadan is still reasonably small scale and before the turtles get camera-shy.

To reach Sipadan, first you need to get to Kota Kinabalu in Borneo. This is most easily achieved on Malaysia Airlines from London Heathrow, with a brief change of plane in Kuala Lumpur; discount fares of around pounds 650 are readily available through discount agents such as the ones which advertise in these pages.

Most dive packages to Sipadaninclude the flight to Tawau, taxi transfer to Semporna and the boat ride. All the dive operators offer only all-in packages, in which dives, tanks, accommodation in beach huts and food are included; weightbelts, wetsuits and the rest need to be hired on top if you are not taking your own.