Since the hurricans last summer, the snorkling in Belize should be better than in Cosumel.
The deep reefs are okay, but most of the shallow reefs in Cosumel were severly damaged by the hurricanes.
I've heard and read rave reviews of snorkling at Caye Caulker in Belize. The Hol Chan Marine Reserve, about 10 miles north of Caye Caulker, is one of the most popular snorkeling destinations. Hol Chan is a natural cut in the reef, about 25-30 feet deep and home for a wide variety of fishes and other marine life. Established in 1987, Hol Chan is the first reserve of its kind in Central America. Just to the south of Hol Chan is another popular dive site called Shark-Ray Alley. Shark-Ray Alley is a shallow cut in the reef where snorkelers are almost assured of encountering nurse sharks and southern stingrays. Often you'll spot dolphins along the way, especially when you head south to visit the manatees and swim off Goff's Caye.