Experts say the extent of devastation caused by the giant waves has laid bare some stark realities: the plight of poor fishing-folk forced to live on the fringe to earn a living, man's folly at clearing natural coastal habitats to chase the tourism dollar, and a modern ignorance of the power of the sea.

All assistant dive instructor Ranjit Sirinaga, 38, in this popular Sri Lankan resort knows is that his livelihood and beloved reefs could be gone forever.

"There was a lot of coral out there, and many colourful fish," he said, staring out to the wind-whipped sea and wondering aloud just how much remained.

"We still have to look and see what happened," Sirinaga said nervously, taking a break from reconstruction work on the tsunami-ruined dive shop where he works. "I hope it hasn't been destroyed."

Visibility has been too bad since the giant waves struck to take the shop's only seaworthy boat out to check on the reef with a dive, but a few snorkelling trips nearer to shore have offered a gloomy glimpse of the consequences.

Three tuk-tuks, two motorbikes, and huge chunks of boats litter just one section of the bay, and coral closer to shore is smothered in sand.

Sirinaga is keen to get back in the water for a full assessment but is baffled by the unforgiving elements.

"Normally in season, at this time, it's a flat, calm sea. But since this happened, the sea is rough. It's different."

Palm-fringed Unawatuna beach is a popular dive location luring 50 to 60 divers a day in peak season. Before the tsunami struck on December 26, it was one of hundreds of resorts around the Indian Ocean coastline that was busy year-round with snorkellers, scuba divers and tourists in search of paradise.

Across thousands of miles of tropical Asian coastline coastline there are similar scenes of destruction. Tourist hotels demolished by the towering waves, entire fishing villages swept away.

"What has made this a disaster is that people have started to occupy part of the landscape that they shouldn't have occupied," said Jeff McNeely, chief scientist of the Swiss-based World Conservation Union (IUCN), who lived for several years in Indonesia and Thailand, two of the countries worst hit by Sunday's disaster.

"Fifty years ago the coastline was not densely occupied as now by tourist hotels."

The beach-dwelling poor and itinerant fishing communities bore the brunt of the disaster. Many had cleared mangroves and started shrimp farms to feed the huge Western market for cheap prawns.

"The mangroves were all along the coasts where there are shallow waters. They offered protection against things like tsunamis," said McNeely. "They were cleared by people who didn't have the long-term knowledge of why these mangroves should have been saved, by outsiders who get concessions from the governments and set up shrimp or prawn farms."

Parts of India and Myanmar, where coral reefs and mangroves remained intact, fared much better when the waves hit than the denuded coastlines of Thailand and parts of Sri Lanka, experts noted.

"Places that had healthy coral reefs and intact mangroves were far less badly hit than places where the reefs had been damaged and the mangroves ripped out and replaced by beachfront hotels and prawn farms," said Simon Cripps, director of the Global Marine Programme at environment group WWF International.

"Coral reefs act as a natural breakwater and mangroves are a natural shock absorber, and this applies to floods and cyclones as well as tsunamis," he said.

Conservationists hope lessons will be learned from the catastrophe which left more than 155,000 people dead and wiped out homes and livelihoods of millions more.

The United Nations Environment Programme (UNEP) has earmarked one million dollars to address immediate environmental needs in the regions struck by giant waves across the Indian Ocean coasts.

But UNEP Executive Director Klaus Toepfer stressed "revitalising local communities and their livelihoods will require rehabilitating and protecting vital natural ecosystems, in particular mangrove forests and coral reefs".

While beaches can be cleaned and deluxe hotels restored relatively quickly, marine life might not recover during the lifetime of coastal dwellers such as dive instructor Sirinaga, analysts say.

"The coral reef system might be totally destroyed. It will take hundreds of years to grow back," said director of Conservation International in the Indonesian resort island of Bali, Ketut Sarjana Putra.

When a tsunami passes, reef structures grind into each other causing extensive damage. In serious cases recovery would be slow as there would be fewer larval animals to repopulate the coral.

Major problems include a loss of fish, displaced by the waves from their habitat, and the amount of silt, sand and organic matter churned into the water which would "smother" vegetation and marine life.

Sirinaga looks out at the mud-covered coral in Unawatuna and expresses hope that it has not been totally destroyed. "Otherwise..." he says, pausing for words, "... well, I'd be very sorry about that."

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