Extend it how far? Hours, days?

The human body, unlike some of our seabird friends and certain other species of animals, cannot concentrate salts and excrete them from our bodies in significant life-sustaining amounts. Yes, we do sweat and sweat does contain salts, but in relatively minimal amounts when compared to seawater. Always keep in mind the simple fact that via osmosis (there are several threads regarding this) that water will move across a semi-permeable membrane, from lower salt (hypotonic) solution to the higher or more concentrated (hypertonic) solution, attempting to equalize the concentrations. The purpose of the osmosis is to try and achieve the isotonic state, which is where the body is able to carry out it’s normal functions.

Drinking any concentrated solution (seawater, urine, concentrated electrolytic solutions i.e. GoLytely, Colyte and NuLytely – what you drink before a colonoscopy to induce profuse cleansing of the colon) will cause water to leave the intravascular space (within the blood stream) and by osmosis, travel across the intestinal wall into the colon, where it will be excreted. If excessive in nature or if fluids (water) are not available to replace lost fluids, dehydration will occur.

Bottom line – DO NOT DRINK SEAWATER - period
Pete