Shark Ray Alley is part of the Hol Chan Marine Preserve between Ambergris Caye and Caye Caulker.  It was annexed there in August of 1999 ( San Pedro Sun Article). Both islands take trips there and it's 15 minute boat ride from town, usually with the dive guides picking you up at your hotel's pier.  It can only be snorkeled as the water is extremely shallow (approximately 8') but is often paired with other snorkeling or SCUBA diving within the preserve.  The site began as a place that local fisherman always cleaned their catch, and consequently, sharks and rays began congregating there.

Expect to see lots of rays and nurse sharks (don't worry, they're harmless and pretty small) swimming around and many of the dive shops encourage it by putting little bits of food into the conche shells on the bottom.  The bottom is sandy there with sea grass so as the time elapses it tends to get silted up and visibility goes down, get pictures right away if you're looking for a good shot.  There are no waves or current at Shark Ray Alley but there can be at Hol chan but the dive shops know when it's appropriate to go out so inquire early and if one day doesn't work, another surely will. 

Nurse Sharks at Shark Ray Alley 

Trips are now combined with Hol Chan Marine Reserve the cut  and they also now include Shark Ray Alley. Since the site is extremely popular many of the afternoon trips complain they dont see the sharks, the reason for this many boats feed the sharks and rays and they eventually get full and move off. Be sure to take the morning trip to get the best experience possible. Some excellent tour operators to check out are Seaduced, Searious and Belize Tours