The Similans are a group of nine small islands, with National Park status
( Mu Ko Similan National Marine Park ) under the care of the Royal Forestry Department, in the Andaman Sea some 40 km west of Khao Lak. They are renowned the world over for their natural beauty and incredible underwater sightseeing vistas at depths from two to thirty five meters.

The word 'Similan' is derived from the Malay word sembilan meaning nine. Each island has both a name and a number. The Thais sometimes refer to them as Ko Kao (or Nine Islands). Counting in order from the north, they are: Ko Ba-Ngu (No. 9), Ko Similan (No. 8), Ko Payu (No. 7), Ko Miang Sam (No. 6), Ko Miang Song (No. 5), Ko Miang (No. 4), Ko Payan (No. 3), Ko Payang (No. 2) and Ko Hu Yong (No. 1). In addition there is a small rock outcrop between number 7 and number 8, known as Hin Pousar or Elephant Head Rock.

The Similan Islands exhibit the greatest variety and sheer numbers of reef fish in Thailand and exhibit at least 200 species of hard coral. They provide a great deal of diversity for the diver. In particular there is a general marked underwater contrast between the western and eastern sides of the islands. The currents to the West have kept the huge, soft coral clad, granite boulders exposed, and free of sand, resulting in dramatic formations, peaks, canyons and overhangs to depths of 30m. These are quite exhilarating dives, and, for some, a guide is recommended to navigate the fan clad passages. On the Eastern coasts the boulders have been mainly buried as the fine sandy beaches slope down to reveal hard coral gardens.

The usual departure point for the Islands is from Thap Lamu pier, which is 10 km from Khao Lak.  Journey time to the Islands is about three hours. An alternative point is from the pier near Khura Buri (north of Khao Lak), which is also the main departure pier for the Surin Islands. One-day excursion and longer tours are available. However the daily boats are somewhat irregular, and the best way to visit the Similans is on a liveaboard boat. 

The best time to visit is from December through to April.

Mu Koh Surin National Park is located in the Andaman Sea and covers an area of approximately 135 square km. The five islets of Koh Surin Neua, Koh Surin Tai, Koh Ri, Koh Kai and Koh Klang were declared a national park on July 9, 1981.

It is an island paradise with numerous beautiful coves and bays, and dense jungle, and covers an area of 135 sq. kms of which 75 percent covers the sea area, and stretches to within 15km of the maritime border with neighbouring Myanmar (Burma). The coral colonies surrounding the islands are the most well developed in Thailand, and are also considered some of the most beautiful. Some of the reefs surrounding the five islands slope relatively gently to the seabed, and are at shallow depths, thus providing excellent snorkelling. 

Ko Surin Tai is home to a small village of Sea Gypsies of the Morgan tribe, who have managed to preserve their primitive way of life; they hold a three day ancestral worship ceremony on the full moon night of the 3rd lunar month.

The best months to visit the Surin Islands are from December to April, when the weather is dry and pleasant, and the seas are calm. March is the hottest month with temperatures reaching a maximum of 34 C. September is usually the wettest month, and November the coolest (24 C). In fact the Park currently closes from mid-May to mid-November due to the rough seas caused by the southwest monsoon.

To reach the Surin Islands, boats can be chartered from beachside operators or via the resort hotels in Khao Lak. Organised tours are also available including for example a combo four day/four night Similan and Surin trip. Several of the major piers, such as Takua Pa and Thap Lamu sub district piers, along the Andaman coast will also offer chartered boats. The serious diver will opt for a liveaboard heading for the world-class dive sites at Ko Bon, Ko Tachai and Richelieu Rock.