I did a dive course last month on Koh Tao. It's a small island in the Gulf of Thailand and is renowned for it's scuba diving and nearby reefs.
The island feature many dive schools (mainly PADI certified, SSI is less common), resorts, and bungalows. The crowd here was a little older than the party backpackers of Koh Phangang, and the one thing EVERYONE is happy to talk about is scuba diving!
A four day scuba course, including basic accommodation will cost around 9000 to 9500 baht, get you four dives, and earn you a PADI certificate in Open Water Scuba diving.
I discovered 'Ban's' resort, which is on the pricier side (9800 baht), but had great accommodation, really friendly and helpful instructors, and good scuba gear. They are the largest dive school on the island, which means you can meet lots of other scuba divers. Ban's has a good reputation and the instructors were great!
I have to admit that I got addicted to scuba diving and did another two day course to get my Advanced Open Water license. In the advanced course you learn to do underwater navigation, photography and nature identification, deep water diving (to 30 metres!), and best of all - night diving! Imagine submersing a reef at night with a high powered torch - the reef comes alive with many fish and creatures that you don't see during the day, and the colours are amazing! We then sat on the bottom of the reef in our group, turned our torches off, and waved our arms around like mad to set off glow-in-the dark phytoplankton.
You can get to Koh Tao by boat from the south of Thailand, or from the larger island nearby - Koh Samui, which you can fly to direct from Bangkok. about 10 years ago
Here's a link to my dive photos from Koh Tao
The beach outside Ban's Resort
The dive pool at Ban's
One of the small bays on Koh Tao
Hope that gives you an idea of what it's like! :) about 10 years ago