There is a ferry that runs between Cozumel and Playa Del Carmen, which is about a 45 minute drive from Cancun. It's been a few years since we took the ferry - I imagine it's about $7 to $10 per person, one way. The ferry runs from early in the morning until well into the evening and takes about 45 minutes to reach PDC. A taxi from PDC to Cancun will probably run you at least $50.00. I'd consider trying to rent a car in PDC, especially if you want to tour Cancun, once you get there.