The Yucatán's Caribbean coast is 380km (236 miles) long, stretching from Cancún all the way to Chetumal, at the border with Belize. The northern half of the coast has been dubbed the "Riviera Maya"; the southern half, the "Costa Maya." In between is the large Sian Ka'an Biosphere Reserve.
A long reef system, the second longest in the world, protects most of the shore. Where there are gaps in the reef -- Playa del Carmen, Xpu-Ha, and Tulum -- you find good beaches. The action of the surf washes away silt and sea grass and erodes rocks, leaving a sandy bottom. Where the reef is prominent, you get good snorkeling and diving with lots of fish and other sea creatures. Here mangrove often occupies the shoreline; the beaches are usually sandy up to the water's edge, but shallow, with a silty or rocky floor.
Inland you'll find jungle, caverns, the famous cenotes (natural wells leading to underwater rivers), and the even more famous ruins of the Maya. Activities abound.