Question

Why is sea water not fit for drinking ?

Answer 1

because it contains salts and other impurities 

Answer 2

It’s not fit for human consumption because of the human osmoregulatory system. We humans are able to derive some mineral content from drinking fresh water and it has potential health benefits (perhaps providing minerals and electrolytes which we might not be getting in sufficient quantities from our diet).

But seawater has so much mineral content - especially salt - that our bodies are quickly saturated with it. While animals such as saltwater fishes have an osmoregulatory method for excreting excess salt and retaining the water that they need in their bodies in the correct volume and ratio to salt, the human body cannot do this and quickly becomes dehydrated. That’s bad!