You won't have any difficulty finding good snorkeling guides on AC. Ask around. Get some opinions from other visitors or hotel/condo operators.

We like the Winnie Estelle. Their current charge is $55 U.S. per person. They run a day trip which includes snorkeling and a stop on Caye Calker. The Winnie is a 66' absolutely beautiful 75 year old Chesepeake Bay Buy Boat. The guys that run it stand on their heads to give everyone their moneys worth. Snorkel trips to the reef and Hol Chan start at about $20 per person.

Good luck!