A coral reef protects the full length of Ambergris Caye. Because the famous Hol Chan Marine Reserve south of San Pedro can become crowded during high season, we explored the waters farther north at Mexico Rocks, a stretch of coral between the barrier reef and the island. We spotted showy stoplight parrotfish, yellowtail snapper, bluehead wrasse, triggerfish and angelfish, as well as a yellowfin damselfish colored as if it were illuminated by black light. A large, green moray eel emerged to snack on fish proffered by our guide, as did a magnificent green sea turtle. Along the bottom, southern stingrays occasionally skittered past. And once, a 6-foot nurse shark glided slowly into view.