We saw all of the advertisements trying to sell you snorkeling activities before we got to the Cancun and Cozumel area, and I am definitely glad we waited to do anything until we got there. You can purchase snorkeling activities over the Internet, but they will cost you between $75 and $90. If staying in Cancun, you can take a bus to the downtown bus terminal and take a connecting bus to the Playa Del Carmen bus terminal, which costs about $3.50. From there, you can take the ferry off of Avenue E, 2 blocks over from the bus terminal. The ferry, which cost $10 at the time, will get you to Cozumel. Spend some time in Playa Del Carmen while you are there. I wish I could have spent some more time there, as it is one of the most beautiful villages in the area.
When you decide to leave Playa Del Carmen, if you do, it will take you about an hour to make it into Cozumel. You can usually book a snorkeling trip on the ferry. Ours was only about $25, plus a $2 port fee, still much less than the $75 they were asking for through other places.