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Humans aren’t adapted for salt water. We use water evaporation for cooling. Our kidneys don’t minimize water usage.

This adaptation is not limited to marine mammals. Jerboas (Desert rodent) get all of their water from food.

Additionally, most marine mammals don’t eat a diet of shrimp. Fish are much less salty than seawater.

There's also a different way of "getting water from food": methabolizing (burning) carbohidrates with oxygen produces CO2 (carbon dioxyde) and.. water. I think that desert mouse eats very dry grain (which does not contain much water), but does not need to drink water because of that by-product of metabolism.

What do the fish drink?

Fresh water fish need to keep salt in and salt water fish need to push it out. They do this with their gills and kidneys.

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