A friend sent me this post and it sent me down a two-hour rabbit hole so for everyone in the comments who are concerned, let me summarize my findings.
First of all, the original quote states that "The hardness of Kalos's water often exceeds 300 ppm." That doesn't mean that every part of Kalos has water with a hardness of >300 ppm. Let's take a look at a water hardness map of France (taken from Reddit).
France has its own water hardness measurement system, but it's pretty easy to convert to ppm (1 °fH = 10 mg/L aka 10 ppm).
So to translate the key in the top right corner:
Green: Fresh water/soft water (Less than 150 ppm)
Yellow: Hard water (150-300 ppm)
Orange: Very hard water (300-400 ppm)
Red: Extremely hard water (above 400 ppm)
Compare that to a map of the US (obtained from an official 1975 US Geological Survey, though more modern maps have similar distributions)
For context, 1 ppm = 1 mg/L, so no conversions are needed here. America also has a lot of areas with hard water, but doesn't have as many areas with extremely high water hardness.
Water hardness is just a measurement of how much calcium and magnesium is dissolved in water. Typically this is associated with areas that have a lot of water flowing through chalk or limestone, which can both release calcium into the water. Western Europe has a lot of chalky soil compared to the US, so it makes sense they would have more areas with higher hardness levels.
But is hard water safe to drink? Well, yes, if you're not made of pipes and metal. Calcium and magnesium are both necessary for humans, so drinking hard water could help you get some of your daily nutrients! (There are some studies that suggest that frequent exposure to hard water can contribute to dry skin and eczema, though.) Does it taste good? From what I've heard, very hard water can have an unpleasant taste, and most people prefer to filter or soften it before they drink it.
If you are made of metal pipes or you're some manner of water carrying appliance, you may require some form of scale removal treatment every so often. Those minerals in the water like to get stuck on things and build up over time. (It's also one reason why you see buildups on your dishes after washing them. It's gross, but it can be easily removed with vinegar or other treatment methods.
So what's up with Kalos?
If I had to guess, its water hardness distribution is probably a pretty close match to irl France's. Coastal and Central Kalos probably have water hardness levels under 300 ppm, with the exception of Route 8 and possibly Cyllage City, since the rocky cliffs by the sea are likely based on the chalk cliffs of Étretat. Compare it to Route 8 from the anime:
Coumarine City may also have higher hard water levels due to the clay badlands in Route 13, since clay can also carry minerals like calcium. Then, much like in the real world, Mountain Kalos would contain most of the areas mentioned in the original post (areas with >300 ppm water hardness).
Now I'm interested in going back to ORAS to look at the other water lore we're given. Reading up on this has been a lot of fun, and now I'm curious to see if or how the facts mentioned in the Slateport Museum mirror the real-world locations they were based on. Anyway, keep up with your local water quality reports, and don't forget to have your wells checked every year!