The cut turned out to be the easiest bit really. (The game is made in unity so I'll use its terminology, but I assume it can be recreated elsewhere) Basically there is a second camera that renders the cow to a render texture. The camera does not see below water level so the resulting capture cuts off everything below automatically. Then I flip this texture and project it to a sprite underwater. That's my reflection. When I merge the cow I combine the original capture with its reflection. you can even see a seam right where the sprites are touching.
Thanks for the feedback!