Underwater, meanwhile, all sorts of marine creatures have helped establish the reefs and the sandy beaches. When you climb up to Sailing Boat Rock, consider the brilliant white sand below.

This beach is in large part a product of diligent scraping and nibbling away by organisms such as the parrotfish - an average individual of which species may excrete more than 16 kilos of sand in the course of a year's lunching on hard corals. And there's plenty here to eat.

The conditions for coral growth are ideal, with a minimum prevailing sea temperature of about 28oC and exceptionally clear waters. More than 200 species of hard coral alone have so far been identified in this area, while these islands have the greatest profusion of reef fish in Thai waters.

In fact, in terms of both marine life and bottom topography, there's more variety than you'll find in most other dive destinations around the world. The Similans has almost everything - coral walls (if we count Koh Bon, to the north, as one of the Similans), big rocks, huge sea fans and barrel sponges, caves, swim-throughs, and plenty of shallows for snorkelling as well. For, although the fringing waters around the islands average from 30-45m dropping down to 70-80m between islands, you'll find coral gardens in as little as six to seven metres.