Speaking from my own personal experience i would pick Cozumel over Grand Caymen for snorkeling. In Grand Caymen do a Sting Ray City/sandbar tour with snorkeling, that's the best of both worlds. In Cozumel there is wonderful snorkeling at the reefs. We did a ships tour to the Palencaar Reef and it was amazing! saw so much marine life including a sea turtle, barracuda and eel. The snorkeling in Cozumel was so easy! there is a gentle current that pushes you along, so you can basically just float along.
I've snorkeled at both (and in several other ports) we measure all of our experiences against Cozumel. In our book Cozumel is a 10, Grand Cayman an 8.5-9 Grenada a 4, etc... You will have a great time in either spot. I agree that the Sting Ray city at GC is a must do. Remember..sometimes weather/wind can make snorkeling difficult in some ports so I'd say do one or both, don't worry about "missing the best one" as both a wonderful.
If you are inexperienced, then go to Chankanaab and snorkel right off the beach. If you want more of a deep water experience take a Columbia or Palancar reef snorkel. These two reefs are great for diving but are fantastic for snorkeling.
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