I've got a spring-fed stream. (Currently a very full stream, glad I cleared the trash grate this morning given the weather). Never dried up even in driest summers. Also some water butts filled with stream water- 4 x 250L- and at the bottom of my Willow chest freezer, I have a dozen 5L bags of drinking water. Being frozen and in the dark, it not go off, and is doing sterling duty maintaining freezer temp as the willow only takes up half the space. (It was put in primarily to maintain freezer temp, then I realised it was also useful spare drinking water, it is in proper water containers).
Our mains water is pumped up the hill, whilst we are at a level where the mains pressure is enough to provide it, if the pump fails/they forget to turn it back on after routine servicing/it trips and doesn't come back on line etc, then the houses above us lose their water. You know when it's happened as the water is very aerated when the pump restarts. Not unusual for it to happen, on average once a month. So I'm conscious of what our alternatives might be if we were ever affected.
(On my "to get" list is a water filter. Or maybe a big pack of activated charcoal to make a filter butt).
We have private drainage, a downstairs loo and a dirty water pump which will lift water out of the stream and plenty of hose. So if we lost mains for a few days, could rig that to pump water into the loo cistern, would use a bucket if it was just a day or two. (There's probably enough head for flow without a pump, but that would involve a lot more piping and would pose other challenges....). If all else fails, I have the composting loo out of my van, plenty soak for it and plenty space to dispose of the liquid bottle contents.
Hot water wise: have a big Maslin pan for jam-making, so could boil a gallon a time- just pop it on the Rayburn and leave it for a while. Fine for a strip wash and washing up.
Not perfect but a big improvement on what we had before we moved here.
GC