Did 3 months in S-E Asia last year. I did my OW cert on Ko Tao, the whole island is pretty much about diving (there's like 42 dive shops on it). People seem to complain a lot about the diving there, but I really liked it (but it's true to most abundant form of aquatic life around the island is scuba diver
). Good place to do your AOW / Nitrox course too if you feel like it. You can also do fun dives from there. Did my cert with Big Blue Diving, good instructors, no problems with the gear, nice boats with food and water between dives, good BBQ at night. If you can hop on a trip to Sail Rock from there I'd recommend it, it was really nice (liked the chimney entry a lot).
I also dived on Ko Phi Phi which was also great (booked fun dives from various dive shops, depending on who was going where since I didn't want to go to Bida Nok / Palong Bay 10 times). The King Cruiser wreck is worth it.
The advantage of going on day trips on boats is that you're back on shore after the dives and can do something else for the rest of day/night. Although if you can, definitely go on a live aboard to the Similian Islands, I spoke to people who did that and they highly recommended it, I couldn't fit it in my itinerary though (stayed rock climbing in Ton Sai too long), so I'll have to go back someday. About Ton Sai / Railey / Krabi. You can also do some diving from there, but the area is more oriented toward rock climbing.
I'm pretty sure Phuket isn't the cheapest place to stay in, although there are probably budget options available. And I skipped the place so I don't know the diving is there (I'm sure it's nice).
If you haven't already I would suggest booking a room in advance in Bangkok so that you don't have to search for a room when you get there (especially if you get there late at night), but then again plenty of people just show up there and find something. And when you get out of the airport, disregard everybody trying to get you into a taxi, continue walking and you'll see the official line of people waiting for a taxi where somebody will ask you where you want to go, give you the price and tell a local driver where you're going and how much he should charge you (this is all so you don't get ripped off). You'll of course get no explanation of what's happening (this is a fairly common thing, you'll get used to it) but it'll happen and it'll be ok ; )
I've found people and other travelers to be very friendly, so don't hesitate to start conversations and get to know people.