More seriously, what if there was a long-term interruption caused by a cataclysmic event such as a Carrington sized geomagnetic storm that knocks out the power grid worldwide, or a global pandemic. Then you'd need to start thinking about a sustainable water source and how to purify that water source. There are many methods for purification including regular household bleach, calcium hypochlorite, iodine, boiling, and robust water filters.
http://water.epa.gov/drink/emerprep/e...
EPA Ground Water and Drinking Water Guidelines:
http://www.epa.gov/safewater/faq/emer...
FEMA Water Guidelines:
http://www.fema.gov/plan/prepare/water