I enjoyed a very nice couple of dives with Mayan Aqua Dive. I found them on TripAdvisor and really liked the fact that they were so trusted by the community. Karla (who responds to most emailed requests for info within an hour or two from her Blackberry) was helpful and informative. There can be special rates available if you pay cash (which also saves you money on the international credit card fees!) so be sure to ask Karla if that might be a possibility.
They have a little kiosk at one of the family-friendly resorts in the Cancun hotel zone. If you are staying at another hotel, they will have a taxi pick you up (free) to deliver you to the hotel. They have decent clean gear. Take the wetsuits they offer unless you really dive hot (temperature, not skill). Despite the water temp being in the 80s, I was a little chilly in a spring suit in February and was feeling sorry for the couple who opted to dive in their swimsuits, they looked extremely cold.
Karla, "Easy" (the boat captain - I think his full name is Ignacio), and our divemaster were very knowledgeable and friendly and very careful divers, taking safety stops and ensuring the group all felt comfortable at all times. I'm taking off one star for two things: first, I was uncomfortable with how much the animals and coral were getting handled by the employees. Second, when I first inquired I asked if there were any dives with no or low current, and was told that wasn't a problem, that they could find a low current area. I found myself in the highest current dives of my life. Our divemaster actually had to swim after to people and he had them hold onto his gear while he pulled himself hand over hand over the coral to get the group back together. I understand now that Cancun dives are higher current than some areas, and I would have dived regardless, but I wish that had been clear to me when I asked.
If I go back to Cancun, I will definitely come back and dive with Karla and the great folks at Mayan Aqua Dive!
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC