To be fair most people in a first world country don’t need to think about water since it’s just “there”, all the time.
But as soon as the electricity goes out the water supply goes out too.
No water supply means no water to drink, with no water the human body die within 3 days, so people will start to rely on any dirty water they can find.
About dirty water, no water also means no WC. I repeat: no WC so no evacuation of feces and urine. Within a few day a big city swill be covered with human excrement. Mixed with no clean water access it means that deadly waterborne diseases will spread extremely quickly.