I can tell you from experience that you don't want to go out and buy cheap equipment. This can ruin a snorkeling trip. Your mask will leak, your fins will rub blisters on your feet, and you will have a horrible time. If you plan on snorkeling more than just in Belize, I would invest in decent snorkel gear. It can make all the difference. A good snorkel mask will form to your face (don't share your mask with others). This reduces the chance of leaking. And a good pair of fins can be quite comfortable. If you don't want to invest in quality gear, the dive shops in Belize have good equipment for rent. Many of the people on our trip opted for this and were pleased with what they got. One other little tip for snorkeling, put a little bit of baby shampoo and water in your mask before snorkeling to keep it from fogging up. This works better than the expensive no fog stuff you buy in the dive stores.
Call Alfonse while you're there and he can take you to the best snorkeling spots. He is very knowledgeable and loves his job.