By air: If you can’t find direct flights to Cozumel, the easiest way to get here is flying to Cancun and booking another flight from Cancun to Cozumel with Mayair, a new airline that bridges to Cozumel. Go to http://www.mayair.com.mx/ to check flight information and book your flight!
ADO coaches are the second option. They depart to Playa del Carmen from Cancun Intl. Airport for $90 pesos per person (subject to change). Tell the bus driver you are going to take the ferry to Cozumel so they can give you directions to the pier. http://www.ado.com.mx/ado/index.jsp
There are two ferry companies that cross to Cozumel which make regular trips almost every hour. Adult single trip fares are around USD $13. Go to www.mexicowaterjets.com and www.granpuerto.com.mx/ for more information on schedules and prices.