Koh Rok Nok is an island ("koh" means "island" in Thai), joined by a sister island, Koh Rok Nai, in the Andaman Sea south of Phuket, off Thailand's west coast. Separated by just over 800 feet, the two limestone islands are part of Mu Koh Lanta National Park.
There is no dock here, and it will be a "wet landing" when you arrive at the beach. You'll have to hop off a tender or Zodiac inflatable boat into shallow water. It's worth the effort; the white sand is powder-fine, and the beach is fringed by jungle, where you'll hear bird calls. (It's rumored there are 3-foot-long monitor lizards lurking, too.)
The area is famous for being one of the best snorkeling destinations in Thailand. There's a large coral reef that's over a square mile in area, and much of it is only 15 feet below the surface. That, and 100-foot visibility, makes it a haven for snorkelers and divers. You may see coral, anemones, eels, parrotfish, clown fish, angel fish, starfish, puffer fish and sea turtles.