Having not read Cable at all, I can only assume that; Deadpool mentions that those that opened up his freezer unit did so looking for rice or water. If drinkable water's that sparse in that time, and Cable and Hope timeslid there, then pee's probably the next safest thing to drink, stupid as it sounds. There's probably considerably less salt content than saltwater, and the waste products might not do you good in the long run, but it's better than nothing.