A small shop on the corner of Sagitario and Osiris in Tulum town, working the cenotes inland — the fresh water, the light shafts, the halocline where fresh meets salt and the view goes briefly oily — along with the reef off the coast.
Cenote diving is not reef diving. The water is cold by Caribbean standards, the visibility is enormous, and a guide is required by law. Bring a wetsuit you would not wear on the reef.