I hate critiquing photos of people I know -- I'm just sure they're going to get upset. Sorry in advance.
What I like:
I love the ducks in motion -- their trails are really just perfect for this challenge, although it's something I probably wouldn't have thought of. Very enjoyable idea.
What I don't like:
The breaks in the water -- I can't tell what they are. Shallow water around grasses? Sandbars? I don't know. Unfortunately, they detract from the ducks which are the focus. Also, the two ducks going in similar but definitely different directions throws off the "rhythm" of the photo. If I didn't know what the challenge topic was, I'd probably not pick up on rhythm being the theme.
Maybe zooming in closer on one duck and focusing the frame on his wake? Or, if multiple ducks were the goal, maybe capturing them without the grasses would help the negative space define the wakes so they'd stand out more?
Very nice idea, though. I do think that the wakes were a great interpretation of the topic.