It's not like hot springs water suitable for a bathhouse would be somehow intrinsically impure. The water just acquires minerals from the same source where it gets the heat, plus from any ground it passes along/through afterwards. I don't really see how Aqua could permanently remove the quality from the water, leaving behind only drinking water behind. But then again, all the main characters are idiots, so they would allow people to take their money away without a complaint.