There is really no good snorkeling off the beach on Ambergris (the reef is several hundred yards out). There are some places close to shore where a little artificial reef attracts fish, and there's a good deal of life in the seagrass, but for great snorkeling off the shore you'd need to go someplace like South Water Caye which is directly on the reef. You might look at the Pelican Beach cottages there. It's a small, remote island with not much else to do other than eat, sleep, dive, snorkel, swim and fish. There are a few other islands on or near the reef that are like that in central and southern Belize.
Belize First Magazine http://www.turq.com/belizefirst/