To answer the original question (sort of), there are two ferry companies that operate between Playa Del Carmen and Cozumel. They seem to charge the same amount but they are not equal.
Take the WaterJet ferry, the blue and White one. Do NOT use the Ultramar Blue and YELLOW ferry. The WaterJets are multihulls that are MUCH faster, and they do not stink of diesel fumes in transit.
Tickets were $8 each way or $15 round trip for either ferry in late June 2003.
Rivera operates a bus from the Airport in Cancun to the Ferry pier area in Playa Del Carmen. The stopping point is actually about a 5 minute walk from the bus terminal to the pier. They sell tickets near the baggage claim area in the airport for $8. The return trip only cost 65 Pesos or $8 when we bought tickets at the terminal.
The trip to/from Cancun / Playa was about 45 minutes each time. The bus is airconditioned, and they had TV or movies playing. All in all it was not bad.
Flying into Cozumel directly would have cost me ~$150 to $200 more, and the times were not wonderful. I will definitely do the Cancun-Playa-Ferry to Cozumel routine again.
Just my experiences and my opinions.
Wristshot