To go to Cozumel you have to take the ferry that leaves from Playa del Carmen. There are ferries departing every hour on average in Playa del Carmen.
No reservation is necessary to take the ferry, just to be present 15 minutes before departure of the ferry and buy your ticket at the box office.
The crossing takes 30 minutes to Cozumel.
Do not wait, book your hotel now in Cozumel