I dont think theres any nude beaches in Thailand...
Nudity is illegal there.... or maybe you can try and ended up in the lock up! LOL!!!
Every now and then you find nude sun worshippers even on Pattaya Beach. Especially the northern end near the Thai Air office.
When you walk the beach from Pattaya to Naklua you will find the occasional naked person.
You get what you pay for, if you are into very old and very large naked ladies on a beach - you will probably find some!