Thailand has rich and varied diving environments, home to colorful coral reefs and an impressive diversity of marine life.

Scuba diving in Thailand can be some of the most spectacular in Asia. The relatively sheltered Gulf of Thailand as well as the Andaman Coast offer a wide range of diving opportunities.

The Andaman Sea forms part of the Indian Ocean and offers some of the best diving in Thailand. In general the Andaman Sea has more extensive coral reefs, but similar visibility as the Gulf of Thailand, with most of the reefs around the 155 offshore islands occurring on the east side of the islands. Most of the diving in this region is organized from Phuket, with a variety of operators offering recreational and technical training, day trips and liveaboard options. Further south the coastline Krabi offers shallow fringing reefs around impressive limestone formations. Some of the best snorkeling sites in the country are to be found here. Offshore, the Phi Phi Islands have grown in popularity as a dive base for the last two decades. The local waters offers great visibility with a rich variety of corals and marine life. The most southerly established dive sites in the country are to the south of Koh Lanta. Here are some 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 70 m, and Hin Daeng, one of the few remaining sites where both manta rays and whale sharks are regularly encountered. The best time for diving in the Andaman Sea is between October and May.

The Gulf of Thailand follows a great parabola from the border with Malaysia on the south coast. The coastline along Thailand is around 1,840 km (1,150 mi) long, more than twice as long as the Andaman Sea Coastline. The Gulf itself is a vast, shallow depression, generally less than 60 m deep and reaching a maximum of 85 m. There are 112 islands in the offshore waters of the Gulf and their shorelines tend to have a better visibility than the shoreline along the mainland.

Some of the best dive sites in the Gulf are in the Western Gulf around Koh Tao, 80 km from the mainland. Ko Samui, further south, has some enjoyable sites on its fringing reefs, and visibility is slightly better than around Koh Phangan to the north of Ko Samui. In the Eastern Gulf, Pattaya has a well developed dive industry, and offers excellent wreck diving and technical diving opportunities. Pattaya has access to some of Thailand's deeper scuba diving waters in the gulf for technical diving and it has a remarkable number of wrecks to explore. Thailand's easternmost diving and snorkeling area covers the waters around and to the south of Koh Chang in the Trat province. This is an up and coming destination and one worth visiting, it is still pretty much un-spoiled and offers shallow waters as well as a huge amount of marine life including the enormous whale shark. If you want to go diving in Thailand from May to September the Eastern Gulf is the better option.