Chankanaab means "little sea" in Maya, and the name belongs to the brackish lagoon at the centre of the park: a small body of water sitting behind the shoreline, connected to the Caribbean through underground channels in the limestone rather than an open mouth. That lagoon is the reason the place was protected in the first place, and it is still the quietest and most interesting thing inside the fence.
Most people come for the other side of the road. Chankanaab's sea frontage sits directly on the leeward shelf, and Chankanaab Reef starts close enough inshore that you can walk down a ladder and swim to coral in a couple of minutes. That is rarer on Cozumel than visitors assume — nearly everything else worth snorkelling here needs a boat. The underwater attractions include a submerged bronze Christ figure, replica Maya sculpture placed on the seabed, and a reef that holds the usual leeward cast plus a decent chance of the splendid toadfish, the species that lives on Cozumel's reefs and effectively nowhere else.
Inland the park runs a botanical garden of several hundred species with replica Maya structures set among them, a saltwater swimming area, restaurants, a beach with palapas, and a set of paid animal programmes: dolphin encounters and swims, a sea lion show, and manatee interactions. Those programmes are genuinely popular and they are also captive-animal attractions. We are not going to pretend otherwise. If that matters to you, you can visit Chankanaab, snorkel, walk the garden and never go near them; if it does not, the operator is a long-established one and the facilities are what you would expect.
The park's real selling point is compression. Ten minutes from the cruise piers, one gate fee, restrooms, food, loungers, lockers, shade and a reef you swim to rather than ride to. For a family with six hours ashore and no rental car, nothing else on Cozumel does all of that at once.
One caution that catches people every season: the current on this shore sets north and it is stronger than the flat surface suggests.