Muted indifference

You simply sigh, sinking back into the water. "It was worth a shot... I think this place is starting to get to me. Losing my legs, having such a quick pregnancy after being assaulted by the drider..." your voice tails off as you shudder, despite the warmth of the water. "These things are making me question whether it's worth continuing. So far, all I have to show for it is a book, a set of clothes that don't like water, and eventually..." You finish by simply gesturing at her cock.

"Oh dear, trust me, I'm doing you a favor. I'm sure that once you try this, you'll realize that it's hardly something to be worried about." She idly strokes her member, a wispy trail of cum floating up into the bath, its waters already beginning to turn a slight white shade. "That outfit of yours has a few powerful enchantments, and it sounds like you've found the most obvious one, which is that it detects vaginal liquid and vanishes in its presence. What you're saying makes sense- it's likely the succubus simply didn't account for water or other outside liquids."

She sits in silence for a bit, as if deep in thought, but when she speaks again it's still not quite what you were looking for. "I can't change my offer, and all I can hope is that your trials go better than they have. There is some aid I might be able to offer, a potentially beneficial enchantment that I could add to the succubi's dress. Though it vanishes visibly and physically, it remains connected to your body, and so I think a charm of infertility would be properly effective, regardless of its appearance."

As if to dispel the hope in your eyes, there's a catch, as always. "I can never work for free, and this is no exception dear. Such a powerful enchantment would even require an additional measure, so to speak. Somewhat fortunately, the two could each be achieved by a single task- I would need to impregnate you myself." A sly smile creeps onto her mouth, seductive and mischievous.