Although I have heard that being topless is illegal in Belize I can find no mention in the laws of Belize.
Having said that, I personally believe that to do so on a beach that local residents frequent is a slap in the face to San Pedrano's. Most of the local population are church going and quite conservative. Half the local girls won't even go into the water without shorts and a T-Shirt.
The young lady parked in front of my apartment on the beach today showed no concern for the fact that she was sitting topless right beside the Catholic High School. I did't mind the view but I'll bet there were some pissed off mothers around.
Charter a boat or find yourself a remote beach somewhere and go for it.
BTW Placencia is even more conservative than San Pedro.