We dove with both outfits mentioned here, and had an excellent time.
Scuba Cancun took us out on their boat over Memorial Day on a reef dive. Excellent sealife... sea turtles, nurse sharks, schooling fish... Boat's a good size, about 5 customers per DM. Bring a signal bouy... took about 10-15 minutes to be picked up.
We did a cavern trip with Manta Divers over Labor Day. The guide, Lief, is top notch. Good 1hr briefing on the way down. Chak Mool was excellent.
If you can afford more than a day of diving, definitely do a boat dive and a cenote!