You can take the ferry from Cozumel, at the main pier, downtown San Miguel. The cost is $18.00 pp, round trip. It takes about 35 minutes to cross. the ferry runs about every 30 minutes, then when you get to Playa del Carmen, you will need to go to the bus station to get a bus to Cancun. It is about 30-40 miles to Cancun. You can do it, but it will be a lot of traveling, and there is much to see and do in Cozumel. Not sure if the bus will drop you off at the terminal in downtown Cancun, or in the Hotel Zone. Probably is more trouble than it is worth. You can check out these websites for some additional information www.cozumelinsider.com and www.travelnotes.cc.
Hope this helps out some.
NCL Dawn (Repo 14 nite) 11-1-09
RCCL Mariner of the Sea (Mexican Riviera) 4-18-10