The coast just North of Tulum city is kinda private, but accessible. We managed to wrangle an incredible place along the beach and had walking access to an open cenote known as Casa Cenote, formerly Manatee Cenote (because they used to have manatees in them).
Next to us was a cool beach hotel called The Tankah Inn, and within it they had an awesome dive outfitter. Lucky Fish Dive Shop...not much of a shop per`se, but these guys have been around and know where to take ya.
Enjoy some of my pictures... http://picasaweb.google.com/MonkeyAnvil/TankahBayTulumMexico