As others have said, the travel from Cancun to Cozumel is fairly easy and inexpensive. The buses out of the Cancun airport to Playa run about every 40 minutes, and are comfortable and cheap. Once you arrive into Playa the bus drops you off about three blocks from the ferry pier, and there are tricyclo guys waiting to help you with your bags if you wish. They work for tips, and a couple of bucks per bag seems fairly standard. I would suggest that you plan on being on the 9am ferry heading back to Playa. That gives you plenty of time to wait for the next bus and get to the airport a couple of hours ahead of your flight home.