@Bootybug Yeah. Logically thinking one might assume that the more foreign substances an amount of water has, the easier it would be for her to manipulate. Or the purer water is, the harder it is for her. Even remotely truly pure water would be difficult to come by anyway, so for story telling reasons regular well water might be difficult already. Something like blood should be no sweat; blood has huge amounts of impurities.
The sword was correct, though: The MC is a huge pervert as it is for wanting to build a slave harem. A huge pervert or just an average Japanese man, that is.