Why will you die if you’re stranded at sea and drink seawater? Anything particular, or just generally bad germs?
Too much salt. How does the body get rid of salt? Through urine and sweat, which are less salty than seawater. If the fluid going into your body is saltier than the fluid coming out of your body, salt will accumulate in your body.
I remember reading that two parts fresh water to one part seawater is better than pure fresh water. You do lose salt, so you do need a little bit of salt intake.
If you drink seawater to stay hydrated, you will die in fairly short order in any kind of survival situation. It would make more sense to, er, recycle your urine, which many have done in that situation.
I would swear I had read a SD column about this but for the life of me I can’t find it. It explained about osmotic pressure and why the fish can live in salt water… I must have been dreaming…
Anyway, as the ancient mariner said: Water, water everywhere and not a drop to drink.