> My thinking is that treating water at home is not that hard. You can easily have one filtered tap at home. It's cheap and easy to retrofit.

I mean, define 'filtered'. A brita-type filter will do precisely nothing for most things that require a boil notice (nor will it do anything for most heavy metal contaminants etc).

In general, "filtered" taps primarily remove chlorine and calcium carbonate, mostly for taste reasons.

Doing actual water treatment at home is not going to be particularly economic or safe.

Water treatment at home isn’t difficult as long as it isn’t polluted with something bad like hydrocarbons. A distiller can be purchased for less than $100. There are many homes in Texas that are using heaters that aren’t heat pumps, so in the winter, it probably is economical if excluding the original purchase price. A reverse osmosis system can also be purchased for less than $500, but requires more maintenance.

> A brita-type filter will do precisely nothing for most things that require a boil notice

Well, it might do something, give a nice warm and humid substrate for bacterias to develop

What about the reverse osmosis filters many homes have?

I wouldn't say "many". RO is expensive and energy-intensive.

The under sink filters I'm aware of cost ~$200 and run passively off water pressure

Under sink filter != reverse-osmosis

Not all under sink filters are RO but some are. There are a variety of options on the market and the prices are much cheaper than bottled water over a year.

Right, but that doesn't mean all under sink filtration systems are RO. RO is a very specific method of filtration

We were talking specifically about RO filters. The one I have in my espresso setup cost a couple hundred and runs on water pressure, undercutting the claim that it’s expensive or energy intensive.

(They are water intensive - I would reconsider that if I still lived in California)

Not really. They aren't common in the US, but outside the US you can buy a $200 unit that will produce RO water a few gallons an hour.

1) There are many, many countries where tap water is not potable. Don't gargle with it when showering, and you'll be fine.

2) Assuming you're in a climate where this makes sense, the easiest source of non-potable water is to simply collect rainwater, so you don't really need a grid for this. In the other direction, use of "grey water" (domestic non-sewage wastewater) for purposes like irrigation is also increasingly common.

> Is there much daylight between unsafe to drink and unsafe to shower/etc.

Have you ever gone swimming in a natural body of water?

> Is there much daylight between unsafe to drink and unsafe to shower/etc. IE, is there such a thing as nonpotable water that's still OK for domestic use?

Certainly. As a kid in 60s/70s, when in India we washed, brushed teeth etc with tap water (were careful not to swallow any!) but only drank water that had been boiled or otherwise treated (the fetish for plastic bottles of water had thankfully not developed anywhere yet). This was visiting relatives, so it's not the case that our guts had adapted to the local intestinal flora.

I think there's an economy of scale of running a single pipe of potable water to each location rather than running (and maintaining) a dual system, which would have "leakage" (both the physical kind, which is normal, but would occur 2X, and the case of people accidentally drinking from the wrong pipe).

The big problem is that point of use treatment invariably misses people. People who don't realize their system broke, those who can't afford $200 for a new one or the extra $5/month in operating costs. The extra public health burden is way more costly than treating all the water. And heavy nonpotable water users are often using gray water or untreated well water anyway!