I think those that are witches should be able to swim without taking phlegm drain to fit in with the theory that witches could float. Might also make for an interesting "witch hunt" idea. Person swim out of a lake you think is to big? They have a lot of phlemgh or are they a witch? Better burn them, just to be safe.
Think someone's a witch? Force them to swim, if they drown, they live! If not pull them out and take their scalp!