I have not been to Cozumel, though many times to Cancun, Akumal, and Merida.
I found this link on locogringo forums that had some good info:
http://www.cozumel-tours.com/cozumel-ferry-schedule.htm
The forums on
http://forums.locogringo.com/forums/default.asp are really very helpful for any travel info inthe Yucatan. If you asked your questions about costs there you would get an answer for sure!
"The primary gateway between Cozumel and the mainland of the Yucatan Peninsula, including Cancun and the Riviera Maya, is the passenger ferry from Playa del Carmen. Thousands of tourists and locals every day take the 45 minute ride to or from Cozumel. It's very easy, and runs nearly every hour. The ferry makes it a piece of cake to go to or from Cozumel on a day trip"
Location: The ferry terminal is located right in downtown Playa del Carmen, about a block and a half from the main square and Plaza Playacar, where most of the tour pickups take place.
Cancun Airport: Most people coming from the Cancun Airport use pre-reserved Airport Transfers. They'll bring you as far as the ferry pier. Then you'll just ride the ferry to Cozumel. Once in Cozumel, you'll take a taxi to your hotel. They're right there waiting for you at the ferry terminal. Very easy. It takes about 45 minutes to drive to Playa del Carmen from the Cancun Airport.
Mainland Hotel Guests: There is a bus that runs from Cancun to Playa del Carmen. You can also take a taxi, or use a rental car. It takes about an hour from Cancun, 15 minutes from Puerto Aventuras, and 25 minutes from Akumal.
Tip: Only buy a one-way ferry ticket. There are two ferry companies, and they don't accept each other's tickets. Doing only one-way leaves you flexibility to ride one ferry on the way there and the other on the way back.
http://www.granpuerto.com.mx/
http://www.mexicowaterjets.com/
Also suggest buying mapchick maps, we use them on every trip - they rock!
http://www.cancunmap.com/
We are going to be down there again Feb 16-24th and can't wait!
Have fun on your journey ;-)