El Cielo. A shallow sand flat off the southwest tip, reachable only by boat, in water so clear the bottom looks closer than it is. The draw is the cushion starfish resting on the sand in three to six feet of water. It is not a coral site; it is a swimming and floating site, and it is usually the last stop on a half-day trip because the boats raft up and the bar opens.
Palancar. The famous one. Big coral heads, channels and sand chutes in 20 to 40 feet, which means you are looking down at structure rather than swimming through it. Snorkel boats work the shallower shoulder while the dive boats go deeper.
Colombia Shallows. Next door to Palancar and gentler. Coral gardens in shallower water, often calmer, and a common second stop.
Dzul-Ha, in front of the Money Bar. The best shore snorkel on the island for most people. Steps into the water, a reef that starts within metres, minimal current on a normal day, and a restaurant behind you. Entry is generally free; loungers are charged.
Sky Reef. Purpose-built for shore snorkelling further down the west coast, with a modest entry fee that typically includes a chair, and gear packages available.
Playa Corona. Cheap, small, local, with reef directly offshore. A good second stop on a jeep day heading south.
Chankanaab. Park entry buys you a sheltered lagoon, easy ladder entries, facilities, and the least intimidating water on the island. It is the right answer for families and nervous swimmers, and the wrong answer if you want wild reef.