Flying into Cancun is both cheaper and easier for me, as there are no convenient flights from Paris to Cozumel (they all involve a one-night stop over in the US).
There are several ways to get from Cancun to Playa del Carmen where you catch the ferry :
- By bus from the downtown bus station (not recommended as it means taking a cab to get from the airport to the bus station).
- By bus from the airport. It's the cheapest alternative, the bus costs less than USD9.
- By shared (USD35) or private (USD50) shuttle booked ahead of time here :
Cancun Airport Transportation - Cancun Airport Transfers - Cancun International Airport, Cancun Taxi, Cancun Shuttle, Cancun Bus
The main issue is to make sure you land early enough to catch the ferry (less than USD15). Departure times vary, and the Ultramar website doesn't seem to update their schedules. To be on the safe side, try to land in Cancun before 5 pm, and for the return, try to fly out after 11 am, which should give you ample time to make the connection.
Lugging your gear is not much of an issue, as you will find tricicleros (locals pushing three wheeled carts) at the PDC bus station and ferry dock. They will take it all the way to the boat for you. In Cozumel, the triciclero can take your luggage to your downtown hotel for less than USD5.
Using the bus, ferry and tricicleros should cost you USD55 round trip per person.
Here a very detailed Internet page about it, with useful links at the bottom :
Cancun Airport to Cozumel by ADO Bus
You can also take a flight from Cancun to Cozumel :
http://www.scubaboard.com/forums/cozumel/307337-flight-cancun-cozumel.html