Preparing is good. I like the idea of a water filter.
If we get to the point of not having a water supply that means that things are so bad that it's basically everybody for themselves.
We would only need a moderate disaster for looting to happen almost immediately, so it would be very hard to get supplies at that time. Supply chains would collapse. Once there is nothing else to loot from the stores, people would turn to robbing each other, home invasions are probable to steal supplies. It would be here that a gun might be necessary.
The lucky ones will have a home many miles away from the cities, as urban center would turn into chaos pretty quickly.