Two choices when a town is under attack

One of the main construction sites you have to consider making in the game is the Pump. The Pump not only provides you with some opportunities to further improve your town's defenses with the strongest traps in the game, but also allows your town to obtain more water through upgrades. Through this mechanism, you'll be able to survive beyond four days even if nobody dies, and with good teamwork you should be able to avoid having to kill each other off to avoid dehydration.

Furthermore, it's possible to find Jerry Cans on the map, which, when purified using a Water Purifier Tablet, provide 2-10 extra rations of water that are immediately added to the town well.

I wouldn't stave off taking your water ration each day unless you're completely unsure with what you're doing with your AP; if your town's organized, you will get that Pump up.

Yes, all towns get attacked at the same time. You don't need to stay out and auto-scavenge over night however. Remember, there are a number of ways to use your AP, including construction, scavenging (sans auto-scavenge), and Workshop conversions.

Since the primary appeal of the game is the social dynamic, there should be plenty of ways to enjoy it without having to auto-scavenge overnight.