Here are some facts:
Chankanaab should not be missed, at least once. We have been there four times. There are several beach areas, including one with a sandy beach and the main area which is sand above a rock bulkhead. You don't climb down over the rocks. You walk down stairs into the water. The beach has loungers and shaded palapas at no extra charge. The snorkeling and scuba gear is available for rent there at reasonable prices. There have been loads of fish to be seen every time we have been there. A grouper came close enough to pet. The reef along the bulkhead is roped off but can be viewed from the rope, and I have seen coral and barracuda there. In the deeper water are various Aztec style figures, a statue of the Blessed Virgin, and a crucifix that must be 10 feet high. None of them can be seen from the shore, only when diving or snorkeling. The dolphin show can be viewed for free from a pier surrounding the performance area, or you can pay to swim with them. My wife and kids have gone to watch several times and loved it. The botanical gardens are beautiful, shaded, and contain replicas, not fakes, of Mayan ruins and structures, and have replicas of Mayan villages. Large iguanas can be seen. There are restrooms and lockers available, as well as moderately priced restaurants and bars. Waiters will bring beverages to the lounge chairs. <http://www.cozguide.net/chankan.html>
We have also been to Playa Sol, a little further along on the road, but it was packed with cruise ship people and a catamaran tour that had a loudspeaker blasting and annoying people. It has about every feature you could want at the site. There are few fish and two or three ceramic underwater figures, but snorkeling is not worth the time.
We have recently been to Mr Sancho's, just a little further down the road, and it has everything Playa de Sol has but there is no admission fee. I did find fish about 30 yards out over some sunken boats and jetski's the put there to attract fish. Very nice and not crowded. <http://www.mrsanchos.com/mr_sanchos/front.htm>
The east side of the island is exposed to more wind and waves and often is unsafe to swim.