It seems the economic system produces more as you buy more. Sell the water to your shops as soon as you get it each cycle buy only ammo. This should produce between 100 and 200 bottles of purified water every few days or so. Sanctuary works well as it has water nearby, which brings me to my next step: build an industrial water filtration system, using at least 6-7 industrial filters with dedicated large generators per 2 filters. Acquire appropriate perks and build multiple weapon emporiums on one settlement for ease of use (five or six is good, but you can make more as you see fit).
