Despite advertising and constant rumors to the contrary, there is no ferry that runs directly from Cancun to Cozumel. You must travel to Playa Del Carmen to catch the ferry. The best way now is to get from the Cancun airport to Playa Del Carmen by way of the bus from the airport to Playa Del Carmen. This is a NEW service that started in July of this year. Once in Playa, you can catch the ferry to Cozumel.
Cost is approx.
There is still the option of taking the island hopper plane from the Cancun airport to Cozumel.