Thailand is a wonderful country with some excellent diving. To the west of Thailand is the Andaman Sea, and to the east the Gulf of Thailand. The best diving is on the Andaman Sea side, both in the south and further north in the Similan Islands.
The best time to visit Thailand is from October to April. Heavy rainfall hits the Andaman coast in May and continues until September. From May to October, therefore, the Gulf of Thailand to the east would be a better option. The best diving on this side is around Koh Tao. Thailand has very clear waters and an average sea temperature of 28C