Ha, technically, you could drink enough salt water to finally extract enough water to live. I say technically because I doubt anyone could actually do it, but if you could force yourself to drink 8 liters of salt water a day, at 1200mOsm/L you could extract 1 liter in usable water/day. The above answer is actually off by a bit, the kidneys can pull enough water out of the tubules (especially the collecting tubule) to bring the osmolarity to around 1400mOsm/L. This is our technicality.
The body is obligated to get rid of 600mOsm/day, so if you add this to the 1200mOsm/l you drink with one liter of salt water, (i.e 1800mOsm/day) you actually lose .28L of water for the first liter of salt water you drink. But since the initial loss included the obligated 600mOsm/day, every litter you drink on top of this would add 14.3% of usable water to the equation ( in other words, for every 1 liter of salt water we drink, we need to excrete .857 liters to balance out the salt we just drank) Add enough, and you finally overcome that .28l that we loose, and start adding positive values to the equation. So 8 liters*1200mOsm/liter= 9600mOsm+600mOsm(obligate)= 10200mOsm/day needing excreting, which at a maximum urine osmolarity of 1400mOsm/liter= 7.08 liters of water lost to rid the body of the excess solute. So, we end up peeing out 88.5% of the water we drink, but, we would technically be able to live ;)