Try Scuba Fred, i used him in July 2011, he is a small dive shop located on the Yankee clipper property in the downtown area near coco bongo and daddy o night club if ur familiar with the area, divemasters are good. He is also an American which shocked me. Dive sites in cancun are so/so. Underwater museum is ok nothing great, the U-boat wrecks are good. I have used Aquaworld 2 times in cancun and they really are a catltle boat outfit, only plus side is their boat departs at 8am but with so many divers i was getting back at the same time as the smaller outfits that went out at 9am as most places depart in cancun at. See if you can do a trip to the Cenotes in Playa del Carmen, its worth the trip. Ive made the trip to cozumel from cancun and playa and its just not worth the time invested because they tend to pawn u off with another cattle boat outfit. Check out some stuff on tripadvisor.com