My suggestion was a little tongue-in-cheek, but it's based on reading about third world sex workers using diet pepsi as a douche, because whatever godawful chemicals are in it act as a spermicide and bactericide

. Mineral water should at worst just be a little acidic - shake it up, spray it in, let it run out. A douche is less wasteful, but one doesn't always have one in the back pocket.
A home-grown vagina will do a good job of flushing itself out, but a hand-made one doesn't have the same capability, alas, and needs some help - whether it's sperm, or a build-up of leftover lube from sex or dilating. But only when it actually needs it, because as you say, a build-up of benign flora is what keeps a girl fresh, not anything out of a bottle.