Azzy
ᚳᚣᚾᛖᚹᚢᛚᚠ
Yes, the rainwater collectors and water-purification tablets provide a mundane way of easy-access to clean water on most days, but clearly Purify Food and Drink (1st Level Spell available to Clerics, Druids, and Paladins) is the winner. It can be cast as a ritual, so it only takes 10 minutes preparation -- so no slot has to be dedicated to preparing it or casting it. One of the guides is a Druid, and there are a few Side Quests that provide an Acolyte or even Priest if the party does not have these resources prepared and planned for.
Actually, clerics and druids must have a ritual spell prepared in order to cast it as a ritual.