Here's some info..
Airport Transfer "Collectivo" or minivan direct from Cancun airport to Playa Del Carmen (approximately 45 minutes traveling time), then Ferry to Cozumel (approximately 40-50 minutes traveling time). Current cost is $15 US per person.
After baggage pick-up, as you walk towards the exit, there will be booths for transportation and taxis. At the booth, ask for a collectivo to Playa Del Carmen. You pay at the booth, receive a ticket and then proceed outside to where the minivans are. They will take your ticket and load your luggage. As soon as one van is filled, the van departs. The vans are readily available all day long.
Express Bus from Cancun Airport to Playa Del Carmen Bus Terminal
As soon as you pass immigration (green/red button) just after the transportation pavillions and just before the outdoors, there is a stand that says Express Bus to Playa Del Carmen. 65 pesos or $7.50 US. Purchase a ticket and there will be greyhound like buses outside. They are scheduled.
From Cancun
9:30am 10:30am 11:30am 12:30pm 1:30pm 2:30pm 4:30pm 5:30pm 6:30pm 7:30pm 8:30pm
From Playa Del Carmen
6:15am 7:15am 8:15am 9:30am 10:45am 11:45am 1:15pm 3:15pm 4:15pm 5:15pm 7:30pm
The drop will be downtown near or at the bus terminal in Playa Del Carmen. Just walk towards the waterfront. You will be able to see the ferry docks. Before the pier, there will be an information booth as well as a booth to purchase the tickets.
Playa Del Carmen to Cozumel (40-45 minutes traveling time)
Then from Playa Del Carmen, take the ferry to Cozumel Island. The ferry boat ride is about 40 minutes on the large boats and the about 50 minutes on the smaller, local ferry. Cost is the same for tourists. It docks right at the heart of the city, San Miguel in Cozumel. Current cost is $80 pesos.
Daily Ferry Schedule (Central Standard Time)
(schedules are subject to change)
Cozumel Island to Playa Del Carmen, Mexico Morning 5 am 7 am 8 am 9 am 10 am 12 pm
Afternoon 2 pm 4 pm 5 pm 6 pm 8 pm 10pm
Playa Del Carmen to Cozumel Island, Mexico Morning 6am 8am 9 am 10 am 11 am
Afternoon 1 pm 3 pm 5 pm 6 pm 7 pm 9 pm 11 pm