He jumps in at the exact same time you do, and the shock collar sents voltage between him and your, I don't know, gold chain, switching personalities in an instant so he becomes the master and you become the dog.

The only way to reverse it is for both of you to jump in again at the exact same time -- but his brain in your body hates water, and your brain in his body is scared of the effects of the shock collar.

Genius!!