Attention, annual closure during the low season (see below)
The Similan Islands offer one of the most beautiful marine sceneries in southern Thailand: turquoise waters, white sands, this group of islands in the Andaman Sea off the coast of Phang Nga province is a popular day trip destination from Khao Lak, Phuket and Phang Nga. Many agencies offer to take you there. The Mu Ko Similan National Park established in 1982 groups 11 islands and has great diving spots. The dense tourism here has led to several measures to try to preserve the environment. The annual closure to tourists during the low season (generally from May 15 to October 15) was the first step but insufficient, it was completed by a limitation of the number of visitors since October 2018 and the end of the possibility to spend the night.
Since the introduction of the visitors quotas, it is absolutely necessary to go book with a local tour agency to visit the Similan Islands because only "approved" boats can enter the national marine park and their numbers of passengers must be declared 24h in advance. The maximum limit is 1,625 visitors per day for trips to the Ko Si Ko Si islands and 1,700 visitors per day for Ko Paet to Ko Si excursions. For diving, the nomber of visitors should not exceed 525 per day. Note that there is also Koh Surin in the same area, which is less known and less crowded than the Similan Islands and you still can sleep in a tent or a bungalow there.