The Phi Phi Islands are situated in one of Thailand's most breath taking areas with limestone cliffs jutting upwards from azure waters. Beneath the surface diving here features caves, swim throughs, walls and caverns. Corals of all shapes and sizes occupy walls including the rare black coral whilst reef fish play close by. Visibility can be an issue here and do expect sites to get crowded in high season.
Here's a great Phuket Blog that also covers other places in Thailand.