We went before the Tsunami hit so my experience maybe a little outdated but I've heard that the Tsunami didn't affect much around these areas.
We went to Similan Islands via Khao Lak, Phang Nga province in Thailand. The mainland office is in Thap Lamu. Boats depart daily from November-May. It's a 3 hours trip each way if you are going on a slow boats, 70 minutes via speed boats.
The water is extremely clear. You can almost see rocks under water looking from the top of the boat. We also went snorkeling around the national parks.
If you like a really beautiful sea, then this is the place to be.