Filter it, basically. Use something along the lines of a millbank bag, or a home made version. As you are boiling the water you need no chemical treatment. Be aware of the area you are in, our area has had lots of chemical plants and in the past chemicals such as mustard gas have been found buried! That is why I will generally carry water, but if in a different area it would be filter and boil.
Obviously don't drink the water from estuary as it will be sea/salt water.
When you have boiled water remember it can be recontaminated easily, tap water still has active chlorine when it comes out of a tap to kill any bugs it picks up along the way, your filtered boiled water will not have this residual protection so be extra clean with your water and bottles.