Disclaimer: I’m not sure if this parallels the lore.
This is basically a two-part suggestion.
First we introduce a thing called water quality (WQ), just as tools have qualities, e.g. butchering quality.
Water quality 7 (unboiled, unfiltered natural water)
Unhealthy to drink. Good chance to cause sickness but a single sip will not cause death.
Water quality 8
Free of micro-organisms, at least free of ones that can’t survive boiling. Might still contain inorganic compounds and small particles, but they’re likely detectable by normal human senses. Likely safe to drink.
Water quality 9
Highest quality drinking water, but not distilled. No micro-organisms, no small particles, no unhealthy compounds. The tap water ideal.
This is where the grand scheme comes in: We’ll have WQ drop in all major water sources slowly over time as the world is being ruined, and as pollutants enter the water reservoirs globally, likely by alien design. Even rain water and ground water get contaminants, and they could be only one level better than the surface water. After the first year, all water sources will have suffered a -1 drop in quality. This repeats every year or even every season. To balance this, the game world will contain water purification technologies, and the player is gently pressured into finding them. Limits could be placed so that it’s not even possible to see -28 water quality or somesuch. We could e.g. have a “Water filter lvl 5-9” which requires at least WQ5 water and is able to purify it up to WQ9. The values could be semi-random, with the more powerful filters being more rare. On a lucky occasion the player might be able to find a filter that just increases the processed water’s quality by 1, and with a little extra energy and time it’s possible to clean it all the way to 9. Or 11, whatever top quality rating is chosen. The purification process could also use the fermentation vat mechanic, so that the player doesn’t have to stand there to wait for a lengthy purification process to complete.
We could alternatively remove any lower limits whatsoever and just have the WQ drop gradually, mercilessly and unstoppably. The player would have to eventually rely on scavenged drinks, and likely be forced to be on the move at all times.
Farming should require some minimum quality water. Otherwise the player might be able to sustain hydration by farming certain fruits and vegetables. If this is combined with the above, even farmers would end up on the road.
Consideration should be placed on the few CBMs that produce water.
The global water quality change could be reversible or irreversible.
Bonus: We’ll introduce a CBM that automatically cleans water up to 11. It will be installed to the torso location, to the back. It shall be called the Spinal Tap Water CBM