I was there a couple of months ago and was surprised about the lack of such a service, at least in the parts I visited...
A flying tour of some of the islands - especially on the Andaman (Western) side of the Thai peninsula would have been fantastic!
I seem to recall Samui having a Piper something for their flying club but I think that was the only light I saw out there.
I guess you'd be aiming your 'product' at the better-heeled tourist, so you would obviously want to be near one of the bigger tourist resorts. Not really sure about the GA airfield situation near these areas (Phuket, for example) so a floatplane sounds quite wise!
You probably don't need me to tell you, but getting around in Thailand can be awkward to say the least, especially if you don't want to 'rough it' on ferries etc. For the better off traveller, an airtour service to/from islands would be very attractive i would imagine.
Bear in mind that the monsoon season could cause problems flying-wise and of course there is the seasonailty of tourism over there. Good Luck
