Yang-Yang develops a sudden interest after the upskirt peek at the girl's white swim suit. He stares mesmerized, follows her to the pool to watch her swim.... and it turns out he's more interested in swimming than the girl. Correct? Or is it a little of both?
And does the swimming thing have a point, other than to give us a funny/weird scare?
bump!
shareHe is emulating her behavior. To be closer to her.
That entire subplot is about his very first romantic stirrings. Contrast that to Ying-Yings awkward "first date", juxtaposed with NJ's reliving his first love with his old flame in Japan. They are all dealing with the same theme but at different stages of life.
It is no coincidence that they students are watching a film about the origins of life. The narrator mentions a bolt of lightning creating the first life on a primeval Earth while we see Yang-Yang's infatuation standing in front of the screen. That moment was a bolt of lightning in his life. His first stirrings of desire for a girl that will, in time, lead him to create new life himself.
The part where he jumps into the pool fully clothed is the director's way of messing with the audience. I think we have been conditioned to anticipate tragedy in films such as this so he uses that to frighten us then elate us when we see Yang-Yang walk in the door, soaked to the bone yet no worse for wear.