Climate
High season in the Similans is from December until April, when the monsoon stays far away. The best period to visit is March, when the winds are calm and the water clear. The national park is closed from May 16 to November 15.
Understand
The park covers 140 square kilometres in total, 14 of those being land in the shape of an archipelago consisting of nine islands. They are Ko Bon, Ko Bayu, Ko Similan, Ko Payu, Ko Miang two adjoining islands, Ko Payan, Ko Payang, and Ko Huyong. For simplicity they are often referred to by their number instead - 1 through 9 starting from the south. Recently, the park was expanded to include the two remote islands Ko Bon and Ko Tachai, but since "Sembilan" is Malay for "nine" this doesn't seem to stick in people's minds and those are not visited as often as the original nine.
Flora and fauna
There is an enormous diversity of fish species. Underwater visibility is the best you will find in Thailand. You will see plenty of colourful fish such as lionfish and clownfish Nemo, and if you're lucky you may spot a bigger one like a manta or even a whale shark. The corals in the area have largely fallen victim to coral bleaching in 2010 and have not yet recovered, though the fish still make snorkelling and diving worthwhile.
On Koh Huyong, the Royal Thai Marines run a turtle breeding facility and access to the island is restricted.
Landscape
The nine granite islands are postcard perfect images of paradise, covered in tropical jungle and equipped with beaches of chalk-white sand. As if this was not enough, the views under the water surface are even more impressive and many people believe this is the best dive site in Thailand. Skin-Diver Magazine has acclaimed the Similans to be one of the ten most beautiful places in the world.
History
The islands were created by upwellings of hot magma during the Tertiary-Cretaceous period some 65 million years ago, then smoothed by glacial ice and the erosion by the sea. The coral reefs are about 5,000 years old and hence the oldest in Thailand. In 1982, the national park was established and now it is scheduled to become a UNESCO World Heritage Site.
The 2004 tsunami left the islands and the underwater landscape almost unharmed, since the waters are very deep around the islands.