When i have been to Cancuun i have dove with Aquaworld and Scuba Freds. Aquaworld is a very large cattle boat operation in my option. They also seem to have their ins at every Cancuun Hotel. They have boats for all your water sports need, fishing too. Scuba Fred is an American who owns a small shop in downtown Cancuun near DaddyO’s night club and the night life scene. He’s located behind the Yankee Clipper hotel both are accessible by bus. I preferred Scuba Fred for the smaller group and more bottom time as well as its a more personal experience. The dive sites in Cancuun are ok, the German U boat is a good wreck dive, underwater statues are ok. If you can try to do a trip to the Cenotes, they’re underwater caverns, located in playa del carmen. Its a bit of trip from Cancuun, as is Cozumel. I have made the trip twice to cozumel from cancuun its a 12hr day. Dont know if it was really worth it but was fun to do. Made those trips with Aquaworld too, check out their websites and tripadvisor. Have fun!