From Thai National Parks website:

Established as a national park in 1981, Surin Islands are an archipelago of five islands in Phang Nga Province 55km off the coast at Andaman Sea on the oceanic border of Burma. The park is covering an area of 141 km² of which 76% is sea. The islands are popular destination for divers from all around the world for it's various coral reef life and white beaches.

The main islands are Ko Surin Nuea and Ko Surin Tai, the other three small islands are Ko Ri, Ko Khai and Ko Klang. Although being outside the national park boundries, Richelieu Rock is part of the park. Headquarters are located at Ko Surin Nuea.

Many websites counts Koh Ban and Koh Tachai to Surin Islands for business reasons and offers trips to these islands only. Both islands are 50km south and belongs to Mu Ko Similan National Park.

Surin Islands are populated with two small communities of the ethnic Moken minority, locally known as "Chao Lay". Mokens are nomadic sea people who live in the Burmese Mergui Archipelago. The Moken population of the Islands is around 150 persons in two villages.

Possibly caused by global warming, 90% of the coral life in Surin and Similan islands died out followed by temperature changes back in 1998 and 2010. The 2010 event called Reverse Indian Ocean Dipole caused a natural "coral bleaching" killed almost most of the marine life. The coral reefs are slowly recovering but it may take decades for full recovery. Both national parks also surrefered from 2004 Tsunami.

Except the villages and campsites, most of the land area on islands are off limits to the visitors. Visitors breaking the rules may expect a penalty of 2000 THB and deportation from the islands.

The entrance fee to the national park is 500 THB for foreigners (children 300 THB) and 100 THB for local tourists (children 50 THB). Mu Ko Surin Marine National Park is open from 15th October to 15th May (confirmed the dates with park officials September 2016) and closed during the rainy season. Some websites misleadingly mentions periods during closed season as "best time for diving", probably to operate to alternative Islands nearby for commercial reasons.

More Information:

Worachananant, Suchai, et al. "Tourism Management in Surin Marine National Park, Thailand." Coastal Zone Asia Pacific Conference ‘Improving the Quality of Life in Coastal Areas’, Hilton Hotel, Brisbane, Australia. 2004.

Surin Islands
from wikipedia: https://en.wikipedia.org/wiki/Surin_Islands

The  Surin Islands  ( Thai : หมู่เกาะสุรินทร์, Thai pronunciation:  [mùː kɔ̀ʔ sùʔrin] ) is an archipelago of five  islands  in the  Andaman Sea , 60 km from the  Thai  mainland. ^[1]  Administratively, the islands are part of Tambon Ko Phra Thong,  Amphoe Khura Buri , in  Phang Nga Province ,  Thailand . ^{[ citation needed ]}

Mu Ko Surin National Park  (อุทยานแห่งชาติหมู่เกาะสุรินทร์) encompasses the islands and their surrounding waters. The park covers an area of approximately 141.25 km ² . It contains the Surin Islands and the surrounding waters. Of the protected area, 108 km ²  or 80 percent is ocean. The park was gazetted as the 29th  national park of Thailand  on 9 July 1981. The park is closed during rainy season, 1 May-31 October, every year. ^[1]