Headed to the Tulum ruins you blow right past the turn on Highway 307 for Tankah. Tankah is a secluded residential area. Aside from vacation rentals, there is Casa Cenote Restaurant, and their dive shop. Casa Cenote Restaurant is an institution in these parts having been around for about 20 years serving up cheese burgers in paradise and BBQ on Sundays.
Right across the street is Manatee Cenote. Now this author has never seen a manatee there, no matter how many margaritas I have drank at Casa Cenote, however people have seen manatee in this cenote. This cenote has a long run. A long run means that the cave water that comes out of the cenote has a long surface pool you can swim, snorkel, and on rough days divers will dive here. On a hot day it is the perfect place to jump in and cool off. There is a nominal entry fee.
If you are a birder, the mornings and afternoons are an excellent time to watch the regional water birds. One of the great birds to post is the Roseate Spoonbill.