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In Cozumel I recommend Chankanaab Park. It is easy to go to with a $10 cab ride, and the children will enjoy all the big geckos that can be seen everywhere in the park. You walk around a little bit, then get to a lagoon, and beach and snorkel area. You see you don't snorkel, but I assume the children will want to swim or play in the sand. Haven't been to the other ports.
