If you are ever in a position where you get stranded and need to wait for help, water is the most essential necessity. To be safe, I would say that you have - at the very least - about one litre of drinking water for every family member for seven days as a reserve that you don't touch unless an emergency compels you to. I would further suggest that you become familiar with collecting water from various natural sources. There are some excellent reference materials available on survival. Over and above studying these materials you are strongly urged to practice some of the techniques at home because it may require some skill, which could save your life in emergencies.