We did the tour off RCL's Nordic Empress last December, they advertised it as:
"Board a speedboat for a one-hour trip along the barrier reef to a popular marine reserve and famous Shark Ray Alley. After you've snorkeled in the incredibly clear waters and had a personal encounter with nurse sharks and graceful stingrays, head to nearby Ambergris Caye for a delicious barbecue lunch on the beach. There will be time to sunbathe or stroll along the sandy beach before you board your speedboat for the return trip along the scenic cayes and mangroves."
I think their price was a little less (about $90 or so). The tour was only so-so. It rained during the hour speedboat trip out. There was no cover on the boat; and everyone got wet and cold. The snorkeling was below average. We didn't see any sharks or stingrays. It could have been due to the weather, time of day, or the fact that swimmers from earlier boats scarred everything off. We went to an older "run down" hotel for the lunch (usual stuff i.e. chicken, rice, beans, and slaw, etc.) on Ambergris Caye. The beach there was poor. The lounge chairs were old, broken, and there wasn't very many of them. We did see a few manatees in the mangroves on the way back.
It problably wasn't worth the price. However, there really isn't anything to see in downtown Belieze City. You need to take a tour. This one might be as good as any. You might see all kinds of sharks. If I ever go there again I might try the cave tubing.
If you want to see sting rays (and lots of them up close and personal) and go to Grand Cayman do the sing ray city tour.