To my knowledge there are none. I saw one beach near the
"upscale" area (not) and it did not look inviting at all, muddy, rocky. I think the beaches people talk about are on islands that are around Belize. I just know I didn't see any and my brotherinlaw and his wife had hoped to find one, skipping booking an excursion, and didn't find one, either. In fact, they headed back to the ship early as Belize is very 3rd world, which means you can't walk around without hawkers pulling on you once you are outside of the ship marina mall area.
Some of the family went on a ruins tour, and enjoyed it very much. We did the zip lining and cave tubing, and had a great time, so there are other thngs to do. Sorry.