We were in Cozumel last year....We never rented a vehicle but I do know that if you do...make sure your not late returning....the ship will NOT wait. The couple in the room next to us went off on there own for the day...and the crew of RCL came to there room...cleaned out there stuff and announced over the loud speaker that unfortunately we had to leave a couple on the island because they did not return on time. They left all there belongings with Immigration and off we sailed..
So just be sure and keep track of time..
Have a great cruise