A couple of observations. First, as to the Cenotes, a lot of divers love them, but a lot of divers find them sterile and boring. I'm in the latter group - I 've done a Cenote dive (years ago), and saw nothing to make me want to do it again. I'd much rather see soaring coral formations, vivid colors, and abundant sea life. Second, while my diving off the mainland (Playa Del Carmen and Akumal) is limited, IMHO, nothing on the mainland can touch the diving available on Cozumel. Staying in PDC, I'd recommend arranging for at least a day of diving in Cozumel. It'll cost you a round trip Ferry ride, and possibly a taxi fare (depending on which Dive Op you go with), and force you to get up pretty early to catch an early Ferry, but worth it IMO. There are several Cozumel Dive Ops that should be able to accomodate you, including Aldora, Blue XTSea, Living Underwater, Dive with Martin, and a lot of others.