I would very highly recommend Beach Divers. They are located just to your right as you get off the ferry from Coz in PDC. If you can, request to Dive with Niko. My wife and I have done about two dozen dives with Beach Divers in the area and fuond them to be extremely friendly, honest, and capable people. They have also shown a level of concern for safety I have yet to encounter anywhere else in Mexico (or here for that matter).
If you want direct contact information I can dig up whatever you want (ie email, phone etc.). Do contact them ahead with your intentions and I am sure you can arrange to meet them at the shop literally less than a block from the ferry. You will of course need to walk a block furhter with them across avenue 5 to get to the car/van.
At the risk of bashing I would suggest avoiding AquaWorld at all costs. We dove Chaq Mool (sp?) cenotes with them, and while our dives turned out quite enjoyable we were appaled at the overall lack of any safety precautions and concerns.
