Why Can't We Drink Sea Water?

Imagine you're stranded on a desert coastline... Namibia's, perhaps. The searing heat drains your body of fluids and your mind slows to a snail's pace. Forget food, forget sunburn, forget your coordinates; your biggest danger here is dehydration. If you don't find water soon, you'll perish, simple as that. As you stand there, you can't help but look at the trillions of litres of water right in front of you. But it's salt water, and you know you shouldn't drink it. You're not sure why, but deep down you know it's off limits.

via Why Can't We Drink Sea Water?