If the Earth ever ran out of water I’d think we’d have colonised somewhere else by then, be it Mars or Europa. Talking about Saturn’s rings…I wouldn’t think they’d be drinkable, as they likely contain impurities! True that its water ice with rocky components but there’s probably some nasty stuff in there!
When you think about Earth though, the phrase is water water everywhere but not a drop to drink! If you had a bucket of water representing all water on the Earth, two cupped hands worth is the amount that we can drink, and then if you put that back in, the water left on you hands is what we can access! Most of the rest of the freshwater is locked up in the ice sheets!