Have stayed at the northern end of PDC (Punta Maroma) - PDC covers a long stretch. Diving there was not too impressive to me in Oct/Nov. Water was warm 82-86º but visibility was not great - 20-40'. Just north of PDC is Puerto Morelos. Diving is in a protected marine park - nice wreck and reef. Visibility was good and water temp at 80º in Mar/Apr. Agree with others about diving in Cozumel over PDC, but if staying in PDC, I wouldn't want to take a ferry everyday I was diving (but you should do it once if you've not been to Coz before) - a taxi ride to Puerto Morelos isn't bad and it's a neat little town.
I'll add a vote for Puerto Morelos. I did a few nice dives there (Fish Market, and the C-56 wreck) and a lion fish hunt with Auquanauts. It was late September, water was warm and visibility was good but the current in places was ripping.
The diving in Cozumel is significantly nicer but the resorts there are a notch or three below what's available in Cancun.