In "Survival - How To Prevail In Hostile Environments" by Xavier Maniguet, the author states that diluting fresh water with some sea water or alternating between drinking fresh water and sea water can extend your supply.

He also states that while drinking sea water may not extend your total survival time, it would allow you to remain concious longer and able to gather water in other ways.

I have no idea if this is true. Doug says on the "Books" page that this theory and maybe some others are not widely shared, which seems true enough <img src="/images/graemlins/smile.gif" alt="" />

I received this book years ago before I was ever even interested in this kind of thing. I can type out a few paragraphs of the relevant sections if that's OK and if anyone is interested.