This is the information that I got from my hostess in Cozumel. Hope it'll be helpful to someone.
From Cancun airport to Playa, you can take the RIVIERA bus.
Departures are at:
10h45 - 11h45 - 12h45 - 13h45 - 15h00 - 16h15 - 17h45 - 19h00 - 20h30.
The trip takes around 1 hour.
It is 65 pesos per person or 8 dollars. It is the cheapest way to get to Playa.
In Playa, you will take the ferry - there is a departure every hour (almost). There are two companies Mexico or Ultramar. The trip will take around 35 mn. Once you are in Cozumel, you could walk (10 mn), or take a taxi for around 3 USD (always agree on the rate first thing!). The ferry ticket is around 10 USD per person. Everybody will accept your dollars. The rate of exchange in the banks is around 10.45 pesos to the dollar. In the shops, they will take it to a lower rate: 10 pesos to the dollar.