There are lots of places to take snorkeling trips in Caye Culker, but I would recommend using the company opposite Da Real Mccaw hotel. It's run by a friendly Canadian woman, with the trips being led by a local called Ian. The trip is really good fun and no where near as scary as it sounds. The trip starts with snorkeling around a reef, where lots of fish and different types of coral will be pointed out. Ian will tell you about the fish and coral and the names of them.
Once the boat arrives at the alley, a short journey, food will be placed into the water attracting nurse sharks and stingrays. Then you just get in swim around and marvel at being so close to these creatures. It sounds pretty scary but once you’re in the water and see how unbothered they are by your presence, it's not so bad. The worst bit is getting in when you can see fins circling the boat. I saw a nurse shark trying to feed from a shell. Ian gets into the water so he’s always on hand and you can get out when you want. Ian lifted some stingrays to show them to us. I took this trip twice and cannot say how interesting it is. There are also plenty of diving trips in Caye Caulker. My friends did their PADI here and really enjoyed it. A few of them also dived the blue hole and loved it, seeing reef sharks, and stalignites.