A good supply of water. Maybe in your scenario the water department might not have power to keep pumping. Even if the pressure only falls from time to time there can be contamination. If it's out you can't even flush the toilet. Surface water might be available, or it might be a long hike. Well water using a hand pump might be available, or not. Catching rainwater or using snow might work. But it would quickly become a big deal to have water to drink, to cook with, to bathe in. I would put a gallon of Chlorox bleach very high on the list of things to have on hand ahead of time. And knowing how many drops per gallon for how long a time to treat water will be critical. A filter would be easy enough to make, and you might already be set up with coffee filters that will do ok.
I learned about clean water when I was in the Peace Corps. I had access to a well where I had to pull rather turbid water up by bucket and rope. I then let it settle for a while, then ran it through a ceramic filter, poured the clear water into a kettle, and boiled it for 20 minutes. Then once it cooled I poured that into jugs. I drank about a gallon a day then as it was very hot, using the straight unboiled well water or rainwater for bathing. I saw lots of water borne disease but only had a few bouts of it myself.
I also learned, while hiding out during a coup, how little water is needed for a bucket bath. You should be able to bathe successfully using a two gallon bucket with enough left at the end to pour over your head at the end. Many people think they need a full bath or a long shower. Doesn't always work when the water is off. But staying clean is very important to prevent rashes and skin infections. We don't think much about that in our society, but we are only a few days away from it if the water goes out.
The prepper subforum is where you want to go for more of these questions. But I probably would never bother to look there as I don't consider myself a prepper.
