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 SCUBA diving is on the Andaman Sea side, both in the south and further north in the Similan Islands.
http://www.scubaplex.com/scubadiving/location/203-scuba-diving-thailand about 5 years ago
Koh Tao, in the Gulf of Thailand, holds the world record for issuing the most PADI licences. The marine life around there is not much, but the island is cool, plenty of young people around, the diving course would be very cheap (around 250 eur for Open Water) and the instructors are for sure among the funniest guys on Earth. about 5 years ago