Rental car/Ferry - Island Hopping

[callvictory]

Hello,

I am coming to Cancun on July 10th. We are going to stay first week in Mayan Riviera (Playa Del Carmen area) and then take a ferry to Cozumel for a week. On the way back to States we are going to spend a last night in Cancun (close to airport area). Please advise how I can rent a car at Cancun Airport and leave it by Ferry dock, and get another car upon ferry arrival in Cozumel, and still pick another car upon return to Riviera Maya. I am trying to pick cars from rentals allowing different drop-off locations, but none of them is specific if it's a ferry dock location. Does anybody knows what Ferry dock approximate address/locations is?
I do need a rental car as we like to explore... I also hope it will be cheaper for me to go to all of those cultural/historic sites on my own instead of buying trips... I also read about car-load ferry in Calica, so I don't have change cars, but I am not sure if it will be possible to transport this way to Cozumel... Please advise. Thank you.
Ellen