Extending from Myanmar down past the west coast of Thailand to beyond the Malaysian border, the Andaman Sea forms part of the Indian Ocean. Many of the 155 islands in the Andaman Sea are uninhabited (apart from communities of Chao Ley -- 'sea gypsies'), and are crowned by lush primary vegetation tumbling down to a shoreline punctuated by idyllic sandy beaches.
Most of the diving in this region is organized from the fashionable holiday island destination of Phuket, with a wide variety of operators offering recreational and technical training, day trips, and liveaboard options. The best time for diving is between October and May.
In general the Andaman Sea has more extensive coral reefs but similar visibility to the Gulf of Thailand, with most of the reefs around the offshore islands occurring on the east sides of the islands, due to the influence of the southwest monsoon; the west coasts tend to have rocky slopes descending to 10-30m in depth.
Towards the northern end of the Thai coastline, a series of granitic outcrops lie in deep, clear water with reefs descending to 25-30m and beyond in depth. These are the Similan and Surin Islands, whose breathtaking underwater scenery and great diversity of marine life have gained them a considerable reputation in the diving community.
Given their proximity to Phuket, it is not surprising that these two island groups are among the country's most fashionable destinations for day trips and liveaboard excursions.
Further to the north in Myanmar is the Mergui Archipelago, latterly opened to foreigners, offering real wilderness diving over unusual underwater terrain, with the added attraction of shark encounters.
The coastline of Phuket itself offers less for the experienced diver, although there are dive sites that able to be (and are) used for training and are a good option if bad weather prevents boats getting to the better sites.
These offshore sites are mostly to the south, and able to be reached by relatively short boat rides of between one and two hours. They are all well worth the journey, but two in particular (Shark Point and Anemone Reef) stand out as being special in terms of the abundance of marine life.
Further south, the coastline at Krabi offers shallow fringing reefs around impressive limestone formations (ranging in size from underwater pillars to whole islands). Some of the best snorkelling sites in the country are to be found here.
Offshore, the Phi Phi islands, with their palm-fringed beaches, have been growing in popularity as a dive base for the last two decades. This is not surprising as the local waters undergo great visibility complemented with a rich variety of corals and marine life; shallow fringing and patch reefs are an added attraction for snorkellers.
The most southerly established dive sites in the country are to the south of Koh Lanta. They feature pristine corals and spectacular fringing reefs in potentially excellent visibility. Here are many of the finest dive sites in the country, such as Hin Mouang, which features the deepest drop-off in Thailand, with depths in excess of 70m, and Hin Daeng, one of the few remaining dive sites where both manta rays and whale sharks are regularly encountered.
Another dive destination in the Andaman Sea that is still in its infancy is Mu Koh Tarutao Marine National Park (just north of the Malaysian border), where dugongs have been seen.