Diving the Riviera Maya is ok, just ok, Cozumel much better. You can take the ferry over in Playa Del Carmen. Suggest Aldora Divers in Cozumel.
The Cenotes are great,, done all of 'em. Recommend Taja Mahal and of course Dos Ojos. Also Grand Cenote, Chac Mool. You can't go wrong with any of them. Just did a "new" one called the "Pit" (recently opened to rec divers). It's a must do, however, because it's new it takes some effort and a rough ride thru the jungle to get to. I guarantee you will not be disappointed.
Recommend Akumal Dive shop in Akumal (about 30 minutes south of Playa Del Carmen) for Ocean and Cenote diving.