Chris described the condition quite well, in reference to cellular osmosis.

Think of it this way; you have a membrane (cell membrane) that is semi-permeable, certain things i.e. ions, proteins glucose, etc. can or cannot move across the membrane, depending upon size and charge.


See:
http://arbl.cvmbs.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html

http://www.purchon.com/biology/osmosis.htm

When you have a solution where there is a higher concentration of salt (hypertonic) then is normal for the human body (isotonic), water will move across the membrane to try and equalize the concentration to make it isotonic.

When you drink sea water the body will try and equalize the hypertonic (too much salt) in the intestine, drawing important fluids from the intravascular system (within the blood vessels), the interstisal spaces (the fluid filled spaces between cells – the cells are bathed in fluid) and then intracellular (from within the cells themselves). This depletion of fluid will cause cells, transport systems, hormonal and thermal regulation to malfunction. This will result in varying signs and symptoms depending upon the amount of seawater consumed, the age, weight and overall health of the individual.

Some animals have biological pumps that can allow them to drink seawater and concentrate the “salts” to be excreted.

There is some connection to the condition (sort of the drinking sea-water, but in reverse) of consuming large quantities of water when one is engaged in activities resulting in profuse sweating.

Pete