Very few people have a source of potable water that does not require treatment on their property. Most of the people who don't have their own water source will probably die very quickly. But not all of them. Some will manage to locate the ones that do have water, and will eventually drive them off or kill them to get it. Even if the water people managed to totally evade detection and remain safe hiding, eventually they would end up like that lone guy in futuristic horror movies who is walking around a deserted landscape without another living human for 1000 miles.
I agree with tipping the odds in your favor by whatever means you can afford. But in the end, in a true society-ending apocalypse, which people here seem to be talking about - I don't think much of anything is going to make a hill of beans of difference. Where preps will make the difference is if things return to pseudo-normal. You have to live long enough to see that happen.
These are just my opinions. Granted, they are not the opinions of a super prepper planning for immortality. I have no qualms with people attempting to do this, I just personally don't see a point to it. I prep for the return of society. Not for an eternity without it.