Phang Nga Bay is a classic Southeast Asian bay - bright jade water, limestone pinnacles and all. A large part of it has been protected as a national park.
Notable islands in Phang Nga Bay include the so-called James Bond Island (it's featured in The Man with the Golden Gun) and Koh Panyee, where you can visit a fishing community built out on stilts across the water. Bear in mind that this community is a Muslim one, so dress modestly.
You can take tours that will drop you at various beaches to swim and snorkel and take you to James Bond Island and Koh Panyee; you can also canoe.
Phang Nga Bay is in the Andaman Sea, between Phuket and the mainland of Southern Thailand. You can hire longtail boats from north Phuket, but probably the easiest way to reach it is on a tour.
