Koh Tao

Koh Tao, in the Gulf of Thailand, certifies more new divers each year than almost anywhere else on earth. Warm, calm water, short boat rides and competitive prices make it one of the best places to take an Open Water course, and many divers stay on to train as Divemasters. The best-known sites are Chumphon Pinnacle and Southwest Pinnacle, granite pinnacles where whale sharks pass through in season (most often from March to May and again in September and October), alongside batfish, barracuda and huge schools of trevally. Sail Rock, between Koh Tao and Koh Phangan, is famous for its vertical "chimney" swim-through. With dozens of dive centres on the island, it pays to compare group sizes, agencies and what is included.