Well, this isn't strictly in Thailand but you can fly here from Phuket (apparently) and it certainly isn't on the primary Thai tourist trail - we are looking to go here in December.
The Myeik Archipelago comprises of some 800 islands scattered more or less parallel to the coast of 480 km long Myeik Peninsula on the Andaman Sea, Southwest of Myanmar. Myeik (formerly Mergui) Archipelago is famous and well-known because of its crystal blue water, untouched coral reef and uninhabited islands clustered close to one another. Some are rocky and some are forested which some have sandy beach. Hilly islands are of limestone stretch-out for hundreds of meters forms caves and enclosed lakes accessible at low tide through rock tunnels. Sea swallow (swifts) build their nests higher which can be very hard to reach, the agile collectors are clever enough to get the nest with risk as bird’s nests are valuable for human consumption. The nomadic salons or sea gypsies sail from island to island, stopping off to repair their boats or fishing nets.
Enjoy about 7 years ago