I did my DM training on Koh Tao in the Gulf of Thailand in the south. There is some really nice diving there - you have the somewhat legendary wall dive of Sail Rock which is a favourite spot for divers to go, although it's a bit further out to sea (which is good as it doesn't get crowded). One of my favourite dive spots is Chumpon Pinnacle - the pinnacle starts at 16m and goes down to 30m-40m. There's plenty to see down there including reef sharks, turtles, barracuda and the occassional whale shark if the season is right. There's a host of other dive spots of varying levels - there's some easy-going fun dives five minutes by boat from the quay which often reveal some nice surprises. There's also more challenging dive sites such as Green Rock - where you're liable to get mobbed in the titan trigger fish pit if you're brave (or stupid) enough to venture in. There's also some decent cave diving, night diving, and a couple of wrecks - although the main wreck I went on was a well-kept secret known only by the members of our dive shop (the course director, Jamie, who's also an experienced techi diver was paid to do some work on it).
There's diving for all levels, and Koh Tao is a really nice island which is near Samui and Phangan if you decide you need somewhere a little more lively - Koh Tao is far less commercialised, a bonus in my opinion.
Master Scuba Divers is the dive shop I trained at - a small but specialised company affiliated with Master Tech. This dive shop can provide all levels of training all the way up to Master Instructor and Techi Deep. Much better than the larger companies such as Easy Divers and Bans who just suck in and spit out reams of OW and AOW divers in the pursuit of large profits - these larger companies don't have the facilities or staff to deal with more specialised training.
When I was at MSD they didn't teach Japanese students purely because they did not have a Japanese-speaking instructor - which might prove beneficial for yourself if you are looking for a job. They were in need of good divemasters - they seemed to have plenty of excellent instructors but DMs were thin on the ground. That was, however, almost a year ago.
As for 'pitfalls' of Thailand the only thing you'll need to do is do a visa run every couple of months to get a new stamp - every ex-pat and tourist in Thailand does it, it's quite normal. This means a trip to the Malaysian or Burmese border, which takes up about a night in travelling on the boat and back. I've travelled a fair bit for someone of my age, mostly in Asia, and I must say that Thailand is one of my favourite countries. The people are pleasant, the weather is usually tres beau (tropical climate so it can rain heavily sometimes obviously), cost of living is very cheap, and the beaches are really something. The only downer I would say is that Thailand is developing quickly what with the tourist market increasing exponentially there. Commercialism is starting to pick up - which has its cons if you're looking for deserted beaches, but if you want the business you're obviously going to need more people there. I wouldn't let that put you off - besides, there's still a fair few places which remain relatively untouched.
Anyway I've gone on long enough - I always do when someone mentions Thailand as I love it there so much.
To contact Master Divers send an email to
info@masterdive.org - ask for Jamie (their course director) and he'll tell you if they need any DMs.
The URL for the MSD website is
www.masterdive.org
Good luck!