I strongly encourage everyone to stock non-perishable food, drinking water and whatever supplies needed to survive at least a month as a matter of course.
While I don't necessarily disagree with the general philosophy, that probably translates into something like 50 gallons of water/person, or about 10 big jerry cans/water bottles which is a lot for most people to store along with everything else for a month.
That's just ~53-gallons each for my three person household.
It's $400 but seems like a reasonable investment in the case things go awry.
Even if you're near a river, having clean, treated water is still useful. It's one less activity you need to concern yourself with in the case of disaster. No need to go out and get water and treat it.
That said, I'm in a seismically active area, in a tsunami zone and pretty much surrounded by salt water so there is value in my situation
There are many long-term water storage solutions. The simplest is a large package of bottled water.
Also I highly recommended supplementing your water supply with a LifeStraw or similar device that can filter water.
https://www.amazon.com/Sawyer-Products-SP129-Filtration-Sque...
(FWIW I haven't used either.)
You are right the Sawyer is better in general.
But what you can do, if you live in an area where any source of freshwater (lake, creek, stream, river, pond, etc.) is available within a reasonable distance (a day's hike?) is learn how to purify water and stock up on the supplies necessary to do so. There are different ways to purify water, but one of the easiest and most effective is still good old fashioned boiling. So if you have a decent vessel for boiling water in, and a means of starting a fire, and some supply of flammable materials, you can boil quite a bit of water.
If you only live near a supply of salt water, or if you live near water sources that are especially dirty / polluted, distilling the water is a step beyond boiling. It's more complicated, slower, and requires more up front prep, but it's not completely outrageous to think of building / buying a distilling unit in advance of a possible failure of the public water system.
The downside of this, of course, is that it requires you have a good supply of firewood or other burnable material, and a place to light a fire where you won't kill yourself with carbon monoxide poisoning.
Chemical purification is another option... and in a pinch, you can get by with plain old (unscented) household bleach.
And, of course, as others have mentioned, there are filtration options. If you go this route, it might not hurt to pre-filter your water (depending on what state it's in) with plain cotton cloth, a coffee filter, etc., to help preserve the life of your filter. The idea is the prefilter step removes the bigger particulates so the "real" filter never sees them.
It will suck eating that everyday, but it will keep you alive.
And if you can't fit a months supply, at least try a weeks supply or more. A case of MREs and a large package of bottled water stashed away in the back of a closet will go a long way in a crisis.