Koh Samui is easy to get to from Bangkok. The dive boats there do most of their dives around Koh Tao. It's going to be way different from California diving, so you'll probably be happy, though the diving on the Andaman Sea side is generally more interesting to me. Are you on your way right now? You said mid-August, and it's already the third week of August.... (or are you planning for next year?)