Diving in Koh Tao or Samui usually expects different marine life like sea Turtles. Usually, you can see them on the closest dive sites around Koh Tao: Twins, White rock, Pottery.

Turtles are a marine reptile, and like the sharks are a very ancient group of animals. They can live over a hundred years and have very few natural predators. There are currently 9 species of sea turtles worldwide, and all are endangered or threatened. Although they breathe air, they live almost their entire life in the sea, coming ashore only to lay eggs.
There are two types of turtle that you can see during diving on Koh Tao: The Green turtle and Hawksbill turtle. They look very similar to each other however have some differences in appearance.
The Green Turtle is the most typical sea turtle, possessing a teardrop-shaped carapace and a pair of large, paddle-like flippers. Despite the turtle’s common name, its carapace color ranges from olive-brown to black in Eastern Pacific Green turtles. The turtle is actually named for the greenish coloration of its fat and flesh.
The Hawksbill has a worldwide distribution, with Atlantic and Pacific subspecies and the Green turtle can be found throughout tropical and subtropical seas around the world, with two distinct populations in the Atlantic and Pacific Ocean.
You can see both types of turtles at many dive sites around Koh Tao and Koh Nang Yuan islands, including sleeping turtles on night dives. If you do see one, make sure to keep a respectful distance and never touch them.