I agree. I happen to think that diving in Thailand is pretty average off the east coast at best. Having qualified OW in Koh Samui in '95 it was a wonderful place to dive. Nurse sharks, turtles and great biodiversity. I dived there for 5 weeks only a couple of months ago, this time based in Koh Tao....the alleged mecca of diving!? but was sadly disappointed I did see a whale shark though learned that it was fished that very same evening, not sure if it was true, very sad if it was. There is an unfortunate undercurrent (no pun intended) running in Koh Tao, no form of centralised governance and so people just literally do as they please. Fishing vessels often fish the shores and Koh Tao, once known as turtle island no longer has turtles, they all left when the beaches were occupied by capitalist regime. Boat captains don't listen to the instructors when decisions are made about which dive site should be visited and I even witnessed one or two being threatened with a sharp pointed instrument when insisting on going elswhere. Consequently dive sites are massively overcrowded, and with relatively little to see when you do dive there is really not worth it.
If you do dive Thailand then take heed in this warning: Always ascend on an SMB and continually look around and listen. Long tails (small transport boats with a long prop shaft) dart about all over and annoyingly close to dive sites, they take relatively little notice of a marker buoy and similarly SMBs, a guy was killed by a dive boat which hit him in the head on surfacing a while back.
Conversely, the Similans, on the West coast are a sublime contrast. Great visibility, lots to see and do (use EAN if you can). Why you might ask is this the case. It is simple....Similans is a national Park, and it is governed and monitored by the Thai authorities, so untouched by the predatory fishermen and long tails don't operate anywhere near!
Ths was not intended to put people of Thailand, far from it, the country is a lovely place and a haven of culture and diversity. Just be selective in where you choose to dive.