hi, there is no water taxi from Cozumel to Cancun, however you may take the federal ferry (fiscal pier downtown) to Playa del Carmen on the mainland (ca. $12) and from there (end of plaza where you arrive in downtown Playa del Carmen) departs a bus every 10 minutes to Cancun downtown (ca. $6). Another way is to take one of the airplanes which costs approximately $250. More info on Cozumel and transportation you find by clicking on my profile...good luck.
My husband and I were recently in Playa del Carmen. We decided to go to Cancun and take the ferry to Isla Mujeres. We used the Servicio Collectivos Van. We paid $3.00 each to ride to Cancun. They go back and forth all day between Cancun and Playa del Carmen. (It helps if you or someone in your group speaks at least some Spanish.) My husband speaks Spanish and that has really helped us a lot. They are located about 2 blocks inland from the bus station in Playa del Carmen.