The waters and islands are nice, very nice. I'm not however a fan of southern Thailand. Northern Thailand is lovely, but it's a bit hard to sail up there. I hope one day to see cruising rules relaxed to the extent that it will be possible to sail freely up along the Andaman coast of Myanmar; before the Hiltons and Sheratons get their teeth into this wonderful section of coast.
The Andaman islands are controlled, I believe, by India and remain off limits for cruisers.
I didn't spend a whole lot of time sailing in Thailand, but fell in love with Phang Na bay and the crystal waters. I based myself in Rebak Marina in Malaysia and thoroughly enjoyed my time there.
"if at first you don't succeed....Redefine success"!