Ko Tao

Shark Bay
photo by flickr user schuey

Ko Tao (lit. Turtle Island) is a small island of 21sqkm in the Gulf of Thailand near Ko Phangan popular for affordable scuba diving.

Wikitravel says:

Historically, it was once the detention place for political prisoners similar to Ko Tarutao of Satun Province, but today it is a great place for divers or anybody who want to get away from the hustle and bustle of Ko Samui and want more than the Full Moon Party on Ko Pha Ngan. Ko Tao is a great place to learn how to dive. There are hardly any currents and a wide selection of dive sites and dive shops, schools and resorts. Activity options are growing outside of diving and the food and nightlife options are some of the best in the Gulf of Thailand.

Wikipedia says:
Ko Tao (also often Koh Tao, Thai: เกาะเต่า, lit. "Turtle Island") is an island in Thailand located near the eastern shore of the Gulf of Thailand. It covers an area of about 21 km². Administratively it forms a tambon within the district (Amphoe) Ko Pha Ngan of Surat Thani Province. As of 2006 its official population number is 1382. The main settlement is Ban Mae Hat. ... read more