From my Cancun Base Hotel (Park Royal Cancun) I visited 3 winter destinations using the local buses and using the ferry and Cancun is still an Excellent choiche for the beaches and water.
Playa Del Carmen is fun for it's 5th Avenue District BUT the sea water (FULL of sea weeds) and his narrow beach full of squeezed people was AVERAGE....
Cozumel beaches are so so compare to Cancun's Ones....Mostly for diving and water sports but the water seem better than Playa Del Carmen's full SEA WEEDS.but Cozumel Downtown is fun like the PDC.
Isla Mujres Island is also really FANTASTIC for his calm sea. As good Cancun's because you have calmer water and it has a nice and fun downtown area for shopping, eating and drinking.
Cancun and the Isla Mujres Island are EXTRA EXQUISITE for beaches and sea water and Playa Del Carmen and Cozumel can be fun for one day trip like I have done this year, BUT not take an hotel in those 2 destinations for an ENTIRE TRIP STAY...The water and beaches are SO SO ....average.
Robert, Montréal, Québec, CANADA