So...you can actually drink sea water (for a limited time) in order to hydrate yourself. The thing is, in quantities high enough to maintain that hydration (as opposed to staving off dehydration), you'd have to drink too much of it over too short a period of time, overloading your kidneys' ability to filter out the excess salt, resulting in hypersalinity. And that's Bad. But, for short periods at the right rate, it is possible.

In the end, it's all about balance: if you go into a situation severely depleted in electrolytes (of which salt is only one option), and drink sea water, it can actually help, up until the balance is restored, and then you start tipping the scales in the other direction.

Homeostasis is key.