Unless you book a shore excursion, Belize is pretty much a strip mall of t-shirts, jewelry and bootleg purse shops. We were informed to not leave the port, so there really wasn't much to do other than walk around the shops. Keep in mind you have to tender to this port, so if you have any motion sickness issues, the water can be choppy. My best advice is to go over on one of the bigger (2 level) boats. They are faster and seem to have less motion. Also, try not to board until right before the boat leaves. The actual ride over is not so bad, it is the sitting in the boat for 20 minutes waiting to leave that has the most motion (especially when leaving the cruise ship - not as bad leaving Belize).
My kids loved this, but it is a 4 hour tour that is in reality more like an hour experience. Most time is spent getting to the location and sorting out the groups. Once you are in the water, it is great, but it is very short. The brochure described additional snorkeling for a price, but it was not offered when we were there. Instead we were taken to more shopping.