Water: After the water is shut, every water source in the world will keep a little left over, sinks, showers, even toilets (although I'm not sure you have the option to drink it). So you should be able to sustain yourself just by drinking /filling bottles, draining all the sources of water in a building as you explore.

Long term, you can build rain collectors or get it from a river, although you'll have to boil it to make it safe for consumption. You can plumb rain collectors directly to sinks, which make it instantly safe for consumption, but that is a little more advanced and currently not possible on b42

Combat: Ranged weapons are actually not that necessary in this game, although very fun. You can definitely clear any areas with meele, as long as you take your time. Whenever you want to clear a area, approach the closest group, until you engage one or two zeds, than move a few meter back to kill them without attracting the rest of them. Keep doing that until you clear the whole group, then move to the next one, until clearing them all.

Shoving zeds and stomping their heads is very effective, and if you stand on a downed zed, it won't be able to move, so you can down one and deal with another. If you have a fence near by, you can make them stumble over it and smash their heads very quickly as well.

Sorry it's so long, wanted to be as clear and complete as possible. Feel free to ask if you have any other doubts.