ugh... I wouldn't boing a fish... remember Futurama? they should have the upside part like a fish and the downside like a girl.
I forget the name of it, but there was a hentai game I played (something to do with a mansion in the woods that trapped some girls there and makes em immortal while they're there) where you banged a mermaid, and it took some thinking but the main character did her. Why not find a mermaid like the one from that game?