I can sum up the nude beach question in one word: "nope" Having said that, remember the phrase "where there's will, there's a way" There are many small islands off the coast of Belize. I am pretty sure you could get one of the boat handlers to take you out, drop you off, and come back to pick you up.
There are also a few secluded spots on Ambergris where you could get adventurous. But just remember that the officials frown on nudity.
Topless is about as far as you can go and that's even frowned at. On the other hand, I don't pay much attention to people who frown alot. I've sunbathed topless on the beach at Ramons but I stay pretty respectful about it...there are kids and parents and lots of Catholics on island. I've been politely asked to replace my top. My policy is "do what you want to do, apologize later."