Drinking salt water increases the concentration of sodium chloride (salt) levels in the bloodstream. To cope with this, the kidneys remove the excess sodium from the body via excretion. If too much salt water is drunk, the bloodstream's sodium rises to toxic levels and the kidney is unable to cope with the payload. Water must therefore be taken from other parts of the body (for example skin cells) to dilute the blood so that the kidney can remove it. This will make you severely dehydrated and could be fatal if enough salt water is drunk.
If great quantities of salt water are drunk, one must drink plenty of fresh water to avoid dehydration.