I love Cozumel and would definitely suggest heading over, if your precious vacation time allows a side trip. However, it should be to experience the island, not for the purposes of snorkeling. I love Coz, and try to get there at least twice a year, but because half my family doesn't dive, I usually end up with a dive trip to Coz and a family trip to Akumal. Yeah, life is tough
That being said, IMO the snorkeling on Coz can't begin to compare to the snorkeling you can find around Akumal, and you really need to snorkel a cenote, of which can only be done on the mainland.
Palancar Reef, and the dive sites off Cozumel are accessed by boat, and they're too deep for snorkeling. My family has, on occasion, gone on the boat with me and done a little snorkeling, but the current can be disconcerting, as can being in the water so far out from land. On days with superb visibility they were able to make out the reef and the divers below. They say the reef and it's colors were pretty, and they could make out a few larger animals, but really nothing else. Other times they really couldn't see anything.
We're headed to Akumal again next month, and because it's my birthday they've agreed to a quick overnight trip to Coz to allow me to get my diving fix
I'm really hoping the eagle rays are still around!
As others have pointed out, there is some really nice snorkeling around Akumal, you won't be disappointed.
Oh, as for opinions on how to get there...we usually just take a taxi to the ferry in Playa. If you're only going for the day, take the bus to the ferry (we can't because we have dive equipment with us). It's clean, safe, CHEAP and reliable. Once on the island, taxis are everywhere. I've never rented a car unless I'm staying for a week or more and then sometimes I don't either. Another option would be to rent a taxi and a driver for the day, about the same as a car. These "guides" can be pretty fun and informative and it lets you enjoy your day...go over to the other side of the island, have a cervesa....enjoy!
If you need more specific info on Coz, ask the questions over in that forum. Those folks will get you set up for sure!