I really really like how you're telling this story.
Especially how you're describing in vivid details the psychological impacts of everything that's happening to her.
That's something I hope to take inspiration from.

A fun idea : He could teach her to swim using some kind of fin attached to a dildo inserted into her, to propel herself. I think it could be interesting to explore her being at the mercy of him basically raping her with a toy and forcing her to self stimulate, to be able to propel and actually swim.