Re: Salt water

     Seawater contains 3.5% or 35,000ppm of dissolved salts, primarily Na+
and Cl- ions.  To serve as drinking water, the salinity must be below
500ppm!
     The best explanation I can think of has to do with the effect drinking
high salinity waters would have on your blood.  If we ingest high amounts
of salt, the ions pass through the stomach lining and into our blood
stream.  Once in our blood stream, our cells have mechanisms to keep the
salts from entering the cells and disrupting the enzymes.  However, part of
this system involves sending water to the environment outside of the cell
to balance the osmotic pressure.  Thus, water is sent into the blood
stream, and blood pressure rises.  If the cells lose too much water or
blood pressure is raised too high, this can be lethal.  I am guessing that
the dehydration is the typical cause of death when seawater is consumed for
a long period.  If you are shipwrecked, and you are so thirsty that you
decide to drink saltwater, dehydration is amplified and death can result
from a few cups of saltwater.
     If you want a more in depth explanation, forward this question to a
physiologist.  Cheers!