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Waterproof map. I was going to ask the same question, but the only answers I could think of is he either wants to bring it on the dive boat to point out sites where he wants to dive or to get a reference of where he is when the boat stops at the dive site.Not saying there's anything wrong with having a map. I'm the sort of cartographic nerd who thinks there's no such thing as too many maps.
What on earth are you going to do with a map of Cozumel dive sites, though?