Tulum is very interesting if you're into Mayan culture and ruins. But as one poster mentioned briefly, you do take a ferry from Cozumel approx. 12 miles across to the mainland and then a bus to Tulum. The ferries are pretty high speed but can at times be a little on the rough side. Not meaning in anyway to discourage you but just trying to make sure you're aware of same.
It's hard to not have a great day in Cozumel !!