The Belize Barrier Reef — a UNESCO World Heritage Site and the largest barrier reef in the Northern Hemisphere — runs just offshore from San Pedro, making it among the most accessible in the world. Hol Chan Marine Reserve and Shark Ray Alley are the most popular snorkeling spots, reachable in 10–15 minutes by boat from the dock.
Shark Ray Alley delivers what its name promises: nurse sharks and southern stingrays congregate at the surface in shallow water. They are accustomed to snorkelers and will swim directly up to you. The visibility throughout the reserve is typically excellent — 20 to 30 metres on a calm day. Book a half-day boat tour with any of the dive shops in town; most depart mid-morning and include equipment.