Cancun has some nice diving.
Ben Mooney at Mayan Aqua Divers is great - he'll get you picked up and dropped off at your hotel (no extra cost) and runs a nice shop.
Mayan Aqua Dive offers Cancun Diving, Snorkeling, Cavern Diving, Cozumel Dives&PADI Courses
avoid aquaworld at all costs.
Ray