Hello Gussy!
I have been to Playa Del Carmen X2, pre-diving, and Coz X2 to dive.
Cancun - haven't been so I can't help- secluded beach?? doubt it
Playa Del Carmen - Stayed at a fancy hotel on the North end of town($200/night). My wife and I could walk North of our hotel and enjoy prestine white sand beaches and plenty of privacy, 10 minutes and you were all alone, shell hunting and savoring every minute. No diving experience there but nearby you will find Xcaret and Xel-Ha. Look them up to learn more...snorkeling, cenote exploration, tribal celebrations and more. Playa offers many great opportunities to dine and experience an International community - 25% US during our 2003 visit.
Cozumel - We stayed at Hotel Cozumel and with the largest pool on the Island +++ we had a great time. Kid's Club for the kids and snorkeling/SCUBA right off the pier/dock. There is plenty of great diving (22 dives of my own) so you can't go wrong!
Beaches here are nothing like Playa. Most established beaches with sand (what sand there is) have a modest entry fee and some ok service. The windward side offers some excellent beach exploration and I recommend renting a car for at least one day to check it out.
Overall - Without knowing your budget or lenght of stay.... The beaches at Playa were the best I have ever seen, Diving in Coz is awesome, Cancun seems not to be far enough away...
So - Playa to Coz or Coz to Playa? Well the ferry goes both ways. If you only expect 2-4 dives, split them between Playa and Coz on your schedule.
If you want more dives - Cozumel will not dissapoint, but take an early day to expore Playa, you will most likely want another!
Torcatdog
I'm going on a trip to that area in April and want to avoid the spring break crowds. Which are would have the least number of people? How bad does it get? Like Cancun? Would it be possible to find a deserted beach?
Thanks!