Water should be one of your biggest concerns in a survival situation. The wet stuff comprises 78 percent of the human body and we Homo sapiens need about a gallon a day to remain properly hydrated. We can only last about three days without water before we die, while we can last three weeks without food [source: Tkaczyk].
In the great outdoors, the best sources of water are springs, head-water streams and morning dew. At home, stock up on enough water to have at least 3 gallons (11 liters) per person which would give you a three-day supply. (Of course, store more if you can in case the emergency lasts longer). And don't forget to boil it for two or three minutes before drinking [source: Tkaczyk].