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 9 islands of which 6 are only open to tourists and has great diving spots. According to the guide of the excursion with which I visited them, the most beautiful snorkeling spot would be that of Koh Payu (Similan island number 7). The main island Koh Similan (Similan island number 8) is also a must for its Sail Rock Viewpoint and the island with the best beaches for swimming is Koh Miang (Similan island number 4). The dense tourism here has led to several measures to try to preserve the environment. The annual closure to tourists during the low season (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 on the islands but there is apparenlty now offers to spend it on a boat.
Since the introduction of the visitors quotas, it is absolutely necessary to go book through 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. But it seems that in fact these quotas are no longer really in force, especially since COVID-19. 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.