In order to function, we – along with our fellow vertebrates – depend on a precise concentration of salt in our blood: 0.9 per cent. Our kidneys regulate this balance, excreting any excess salt in urine. Drink seawater and your kidneys struggle to produce enough urine to flush out the salt, leading to rapid dehydration. Marine animals have evolved different solutions to this problem. Fish get rid of salt through their gills. Seals and whales have extremely efficient kidneys, producing very salty urine. And most sea mammals simply avoid swallowing salt water, getting fresh water from their prey instead. These adaptations, however, come at a cost: it takes a lot of energy to fuel super-efficient kidneys, for example. It seems the evolutionary trade-offs involved in adapting to salt water weren’t worth it for us.
Answered by Alex Cheung.