Redundancy is a must especially with a key element of survival like water. Keep a jug in the car and multiple ways to purify water. I like to have water on hand, a purification device like a life straw or chlorine tabs and a way to boil water. Since fire is also a key element and already having the redundancy of multiple ways to make fire you can't beat having a small metal cup or pot to boil it in.
That said I have a camelbak, Lifestraw, chlorine tabs and a small metal cup(for cooking or boiling water). Straw and tabs are packed into the cup saving space and its relatively light.