You can fly into Cozumel using one of the many airlines that fly directly (Air Canada, Air Transat, Delta Airlines, Frontier, Interjet, Sun Country, Sunwing, United Airlines, US Airways, West Jet, Xtra Airways) or fly to Cancun and then either fly or take the bus and ferry to the island.
From Mexico City Airport: You can take a flight directly to Cozumel airport. (Time: 2 hrs)
From Cancun Airport
You can either take a flight via Mayair (Time: 15 min) or take the airport vans or bus to Playa del Carmen (Time: 50 min).
From Cancún Down Central Station
You can take the bus to Playa del Carmen from the main bus terminal (Time: 1 hr)
From Playa del Carmen
You must take a ferry ride, ferry runs from around 7am till 10pm. Check schedules upon arrival to Playa del Carmen. (Time: 45 min)