With over 20 million visitors a year and very little regulation, “unspoilt” is becoming more and more difficult.
THe main problem is quite logical – if a beach is REALLY beautiful then there will be virtually no restrictions on what a wealthy develpoper can get away with……the inevitably result is that ALL the top grade coastal attractions are overrun or destroyed.
THat leaves the sort of places that as yet have not attracted enough people to warrant the attentions of the greedy developers whose only criteria is quick and easy profit.
To this end you are better looking at in land areas, national parks etc.
I agree that Kanchanaburi Province and the Western Forest complex offers a great opportunity to see bits of unspoiled rural Thailand before it finally disappears.
Also towns mentioned like Prachuap Kiri khan have not yet been overrun by foreign tourists as their beaches – although quite respectable – are not 5 star and therefore the target of untrammelled development.
The further away from the main holiday centres you go, the more likely you are to fid towns that have not been ruined by rampant tourism.
The coastlines between Hua Hin and Suratthani still have quaint places as do the coastlines between Phuket and Ranong, Phuket and Malaysia (mostly) and Sattahip and Trat.
these 3 coastlines are relatively unfrequented by foreign tourist hoards (as yet) and although they may not be the “best “beaches in Thailand you may find the lack of crowds and reasonably priced local food etc. more than enough compensation.
[BTW – all these places are popular with Thai vacationers – on public holidays and week-ends]