Ambergris Caye offers superb snorkeling. To enjoy it best you'll need to take a short boat trip out from shore.
You can snorkel off the beach at your hotel or anywhere on the island, but you don't usually see as much. Some hotels work hard to keep their piers as appetizing to the fish as possible. It can be quite pleasant to float around and see what you can see off shore.
The three most-popular areas for snorkeling from boats near Ambergris Caye are Hol Chan Marine Reserve, Shark-Sting Ray Alley and Mexico Rocks. A visit to Hol Chan and Shark-Ray Alley to the south of San Pedro are usually combined into one trip, while Mexico Rocks to the north of town is usually a separate trip. Boat trips to either of these destinations cost around US$14 to $18 per adult, which usually includes snorkel mask and fin rental and a guide who will go into the water with you and point out the sights. Kids under about age 12 go for half price. There is an additional US$2.50 per-adult fee for admission to Hol Chan Reserve. The fee, which goes toward preservation of the Belize's parks, is collected at the reserve by rangers in power boats, so be sure to bring your money.
These snorkeling trips usually last two to three hours. Typically, snorkel boats go out once in the morning and once in the afternoon, more frequently during busy periods. A couple of dozen dive and snorkel operators offer snorkel trips. It is difficult to recommend one over another, as the quality of the experience depends on who is your guide on a particular trip and also the weather and sea conditions. However, see the list of recommended dive shops in the dive section (http://AmbergrisCaye.com/pages/town/dive.html) of this website.
Note that boats cannot always go out, due to wind and weather conditions. When there's a strong wind or during "Northers" in the winter, snorkeling is unpleasant at best and could be dangerous.
Night snorkeling trips also are available, at around US$20 per person. Night snorkelers may see lobster, eels, octopus and other creatures.
The Hol Chan Marine Reserve (http://AmbergrisCaye.com/holchan/), about 4 miles south of San Pedro, is a 5-square-mile underwater national park established by the Belize government in 1987. Because fishing is prohibited in the reserve, there is a considerable amount of sea life. At the cut here, you may expect to see large groupers, nurse sharks, sting rays, moray and other eels, spadefish, schoolmasters and other fish. Much of the bottom is sandy, but you also will see bright coral. Click here (http://AmbergrisCaye.com/pages/town/divesites.html) for more dive sites.
Catamarans and other vessels based in San Pedro visit some of Belize's other cayes and atolls, on longer trips for snorkeling and picnics. You typically pay US$35 to $75 for these trips, depending on the length of the trip, where you go and whether lunch and drinks are included. San Pedro also has several glass-bottom boats which offer trips for around US$20.
It's a long way to go for snorkeling, but Belize's atolls offer good snorkeling in shallow water around patch reefs. Day snorkel trips to Lighthouse or Turneffe atolls run about US$100, including lunch and snacks.