Koh Ha
Undoubtedly, the diving highlight at Koh Ha is the series of fascinating caverns which are located at Koh Ha Yai, the largest of the group of islands. The caverns have large entrances and are safe to enter. You can surface inside and see the ancient stalactites which hang from the cave ceiling. Sunrise is a magical time to dive here as the sunlight creates a gorgeous emerald colour.
Another big attraction here are the swim-throughs. You can access the main cave by entering the swim-through to the right of the main cavern entrance. Once through the entranceway, drop down a little and then make a sharp left to squeeze out through the exit point. You'll find yourself back in the main cavern's chamber!
Koh Rok
The west side of Koh Rok is comprised of a hard coral reef that slopes gently downwards. There is an abundance of colourful reef fish here as well as plenty of moray eels and octopus. Look closely for the numerous different nudibranchs. Blacktip reef sharks can be seen in the early morning and again at dusk. Watch out for Kuhl's blue-spotted stingrays partially hidden in the sand on the sea bed, the most common type of ray found at Thailand's dive sites.
On the eastern side of Koh Rok you'll find lots of soft corals with large Gorgonian fans and sponges. This attracts plenty of turtles and you can get quite close to them, especially while they're feeding. The point of the island has fairly strong current so pelagic fish are often seen here including mackerels and cobias.