There are a lot of things that are illegal in Belize. There are also a lot of VERY remote places you can sunbathe nude if you so desire and the odds that another living human being will know you are doing it are rare.
I seriously doubt that the San Pedro police would take the time to get out of their chairs, go search for a boat, pay for the fuel and drive all the way to Rocky Point to arrest me for sunning nude on a boat. LOL
There is a place for everything. I wouldn't suggest doing it in town. I think respect for families and local rule is in order in town.
I don't however see anything wrong with going to a private place.