San Pedro is one of the better-positioned dive bases in the Caribbean. The reef wall just offshore offers good diving at all experience levels, and day trips reach some genuinely world-class sites. The Great Blue Hole — a 300-metre-wide marine sinkhole 70km north of town — is Belize's most famous dive: dramatic stalactites hang from the cavern ceiling at depth, and reef sharks patrol the rim. It is deep (the interesting part starts at 30m) and best suited to experienced divers, but the spectacle is unlike anything else.
Closer alternatives include the Turneffe Atoll, roughly 30km offshore, which offers wall diving, shark encounters, and healthy reef. Several certified dive shops in town — including Amigos del Mar and Belize Diving Adventures — run courses, rent equipment, and organise day trips. A two-tank dive to local reef sites typically costs around US$100.