For all that he feels towards water, the hate and abhorring disgust, not a single iota of it is directed towards her. He knows that she likes the water, and wishes him to know and like it as well. It's like him wishing Rachel to know and like the different tastes of food. It's something she wants to share. So there is no forgiveness to give for dunking him.

Also, it could be that revenge may be inacted when he finally does catch her. Turnabout, they say, is fair-play.