Well... I think only a true, die-hard fan would want soap that has used bath water (there's definitely an ick factor). In which case, I don't think it matters what is in the product. If the consumer isn't a fan, why would they want it? There are so many competing products without the ick factor.
As for feminism... yeah, she can do what men can do. But it doesn't mean that women have to buy into ideas they think are less than stellar, even if the proponent is a woman. If a man is selling his used bath water and can get people to buy it, good for him. Still doesn't mean it's a good idea.