It's one of those 5-element wuxia stories, and water is mostly for healing and weak attacks, since the other four elements can easily disperse/absorb/overpower it. And it's especially bad for women, because due to the male/female yin/yang thing, a male practitioner can leech it away, making them very desirable for, ah, short-term power levelling (and since high powered figures in wuxia worlds are practically immortal anyways, healing is useless). So, when it turns out that the daughter of one of her friends is a pure water user, she decides to train her and offer up a little tidbit of advice about why water users are OP.