How do I cool my water bottle immediately for an emergency use?
Ammonium nitrate, calcium ammonium nitrate, and urea are each used in instant-cold packs. The dissolution of any of these compounds in water is an endothermic process and will thus cool your container of water.  

In a practical sense, it doesn't seem likely that you would sacrifice some of your water so the rest of it could be cold in an emergency situation. Of course, the water you use for cooling does not have to be clean and potable.