There is a limit to the concentration of salt in drinking water that the body
can handle. At the levels in sea water, if you drink enough of it to satisfy
your need for water, the salt will be more than you can handle (I'm not
certain what the various symptoms are, but delirium wouldn't be too surprising
from the effects on membrane ion potentials). Too much salt will definitely
induce nausea, which is not a good thing when you are thirsting.
Sea water is only somewhat more salty than the body can handle, so in a
survival situation, you can double your available water by diluting sea water
50-50 with pure drinking water.
I would guess that it wouldn't give you quite the good of an actual doubling
of the amount of pure water, as the body has to fight to deal with the extra
salt, even if it's within the tolerance level.
Kyle L. Webb Dept. of Physics + Astronomy
kwebb@gkar.phys.unm.edu University of New Mexico