Snorkeling ? from a newbie

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Hello, I would like to ask what makes a location good for snorkeling? I realize that reefs are good places, and have read that sandy beaches are not. Looking at a map of Cozumel I see all the reef locations, but have also read that the is excellent snorkeling at El Cozumeleno resort which does not show a reef near it on the map (NW corner of the island). Just wondering more specifically what makes a place good or not. I've only snorkeled once and that was at Xel Ha in Riviera Maya. Thanks!