Actually you can snorkel off the shore, especially around docks. The seagrass is home to a lot of sea life (that's one reason it's there) including rays, small fish, etc. But the best snorkeling is around the reef. Lots of boats going out all the time. You'll see them once you get to San Pedro.
My teenage son enjoys San Pedro. It's safe for him to walk around town and "explore." Also, tons of things to do on day trips to the mainland. (See my list from Belize Book of Lists 2000 below.)
Belize First Magazine http://www.turq.com/belizefirst/
10 Best Things to Do with Kids
Belize is best for kids old enough to appreciate nature and to entertain themselves outdoors. There are no fast-food restaurants, malls or video game parlors in Belize, but many hotels in Belize City, San Pedro and a few other places do offer cable TV with all the U.S. channels.
# 1 Visit the Belize Zoo
Hands down, this is Belize’s favorite place for kids of all ages.
# 2 Beach it
All over Belize, you’ll see local kids swimming off piers and beaches. Belize’s beaches may not be as wide and sandy as some, but kids don’t mind. They’ll have a ton of fun.
# 3 Hang out with Belizean kids
About two-thirds of Belizeans are under age 21, so kids are everywhere.
# 4 Go snorkeling
Kids as young as 5 or 6 can enjoy the wonders of undersea life, but don’t push them too fast. If you’re with a guide, ask for water wings or a life jacket for the little ones, as some currents through the reef are strong.
# 5 Climb a Maya ruin
Young’uns go where adults are too tired to tread — they love running up and down the temples and other structures at Altun Ha, Xunantunich, Lamanai and elsewhere. Kids may not think much of the history, but they’ll enjoy the exercise.
# 6 Take a guided nature hike
Most Belize guides are highly knowledgeable about wildlife and flora. They’ll open the eyes of your kids to all kinds of natural wonders. Tarantulas, crocs, snakes, wee-wee ants and monkeys seem to hold more appeal than bird for kids. Young people seem especially to enjoy seeing and hearing the howler monkeys — there are troops at Community Baboon Sanctuary, at Lamanai, in Cayo, around Monkey River and elsewhere.
# 7 Hit the river
Most kids will get a kick out of swimming or tubing in one of Belize’s many rivers. Resorts, lodges and tour operators offer river trips on the Macal, Mopan and other rivers by tube, raft and canoe.
# 8 Visit a butterfly farm
Belize now has six butterfly farms and centers (see page 70). Kids will enjoy seeing one, though after that their interest may wane. For kids, we especially recommend Tropical Wings near San Ignacio.
# 9 Splash in a pool
A swimming pool makes any day more fun for kids. Increasingly, as part of the upscaling of Belize tourism, Belize hotels offer swimming pools. (See our list of the best ones, page 55). The ones at SunBreeze and Chan Chich are especially great for kids.
# 10 Make friends with a quash
It may not be 100% “EC” (Ecologically Correct) but a number of Belize lodges have once-wild animals as pets. Often, these are animals that have been injured, and the managers are nursing them back to health. In other cases, such as with iguanas, the lodge may be raising these in a controlled program. In any event, the opportunity to see monkeys, small cats, coatimundis, iguanas, parrots and other animals up close and personal is a treat for kids. Among the hotels with animals around are Lamanai Outpost, Jaguar Paw, Banana Bank, Pook’s Hill and Hotel San Ignacio.