If you want to dive the cenotes, I highly recommend Dennis at Aquanauts/Diablo Divers (
http://www.aquanauts-online.com/). We did a cavern tour with him last month, and he's a great diver and guide. I am heading back down as soon as I can to take the cavern class with him. He's located in Puerto Aventuras, but you'll have to head down in that direction to get to the cenotes anyway. We also did some ocean diving with his shop including the tortugas dive and were very happy.