So I'm not sure if this is some kind of bug, but when I beat the mermaid girl I couldn't leave the area, nor could I 'capture' her by holding up, so I restarted the game, as I thought she wasn't in my selectable girls in the menu, however a friendly version of her appeared among the other random girls I'd collected so far in the waterfall area, I fucked and captured her, upon still not finding where to go I went back to her boss room and she was there again, and I had to fight her again, only for the same thing to happen now. :/