Fires, floods, earthquakes, and a variety of other situations can impact your clean water supply. Plan on having at least one gallon of clean water on hand per person per day, for both drinking needs and sanitation (bathing and washing dishes).
You can purchase gallons of purified water at your local grocery store, or, in a pinch, you can wash and reuse empty soda and juice bottles and fill them with tap water.
If you’re making a survival kit for travel or evacuation, it’s probably not reasonable to carry more than a gallon of water with you (and even that might be a stretch), so you might want to consider using a portable water filtering straw, like the LifeStraw. Filtering straws are small, packable, and lightweight, so you can take them pretty much anywhere—and you can use them just like straws to drink from any water source.