The first thing to know is that there are 2 companies that run these ferries between Cozumel and Playa. They leave on the hour and the two alternate their trips, so one goes out on odd hours, the thereon even hours. This means you should NOT purchase a round trip ticket because you may get stuck on Cozumel for an extra hour if the return time is not right or you miss the return you wanted.
This happened to us. We booked round trip and wanted to leave on the 4pm back. However we got really slow restaurant service and missed the 4pm and had to wait for the 6pm. Since Cozumel was nearly shut down (Sunday in the slow time of year) there was very little to do!
Each trip is about US $16 so round trip is 32 per person. You can get special prices by swinging a deal with snorkel or diving tours. Also, these tickets are fairly well used so if you end up with extra tickets, you COULD sell them fairly easily around the dock areas.
The trip takes about 30-40 minutes and can be a little rough. The two companies are similar but the Ultramar boats have bars on them and we hear that the Mexico Jetboats do not. Also the Ultramar boat had live entertainment on deck but I’m not sure if that was a normal thing.
Cozumel is a big port for the cruise lines and so the economy is based around that and snorkeling/diving. If the cruise line boats aren’t in, the town slows way down. Add Sunday in the slow time of years and nearly everything was closed down.