One of my recommended things to do in Cozumel is joining a snorkeling boat tour. As soon as you set your feet on the docks, you will be approached by local vendors who will try to sell you a one-day excursion. Take it easy, visit different agency stands, and choose the one that you like the most.
Boat excursions last a couple of hours. During your tour, you will be taken to different snorkeling sites, where you will see the local reefs and fish.