Having access to good water sources are an important prep. I have a creek right across the street from my house; it is probably 50 feet from my front door. Up the hill from my home, there is a natural spring and it has always ran a small stream of water down the hill by the side of my home. I have lived here since 1987 and until this year, the spring has always been reliable. I have spoken to older residents that say they can remember that spring running 50 and 60 years ago.
This year the spring has dried up. The creek across the street still has water in it, but less than normal for this time of year. Other sources of surface water are reduced because of the drought. I think the creek is large enough to keep running all year even with the drought. But a lot of people would have access to it.
If the water system failed and knowing the habits of many people, I feel that the creek would have dirty diapers floating down it in just a few days. By the use of Google Earth I have found the location of all the swimming pools near my home. Now these would by no means be my choice of potable water, but they can be used for sanitation and to water gardens. Swimming pools have some potentially serious problems due to the chemicals that are added to them. See my post Is Swimming Pool Water Safe to Drink?.
When you go to collect water, avoid areas where high levels of salt may be found such as alkali pits or salt water. Cattle ponds and creeks where water runs off heavily fertilized fields should be avoided if possible. These may have high concentrations of nitrates. In the area where I live, you have to be a bit careful around old mines because of mercury contamination. Ponds in golf courses are normally heavily contaminated with fertilizer runoff.
Knowing the potential water sources in your area lets you plan ahead. If you have a stream or creek, it is always nice to know what is upstream.
Good water storage can help you survive the first few days when there will be confusion. I would recommend you store enough water to last for several weeks. This will give you time to survey the situation and make plans without having to rush into anything.
Hopefully you have a good functional water filter in your preps. Regardless of who manufactures your system, you want to extent its use as long as possible. So use the cleanest water available. If the water is, dirty and contains sediment and plant debris, I would filter as much material out of the water as possible prior to running it through my filter. This will help extend the life of the filtration system. This can be done in many different improvised ways, some as simple as straining it thought a cloth. Homemade sand and rock filters can be used in conjunction with your good filter system.
Get to know the water sources in your area and at the same time get to know your neighbors who may control access to them.