Since swimming was involved, it was natural that they had to wear swimsuits.
This had been written in the script a long time ago, including the scene where the male lead and his buddies went to admire the beauties (mainly the female lead).
This was also a key scene in the movie, which they would probably use for the poster later on. To prepare for the swimsuit shoot, Xia Siyu had strictly forbidden Bo Yan from touching her the night before. She was worried that a slight lapse in his attention could leave a mark, which wouldn't be good for the film.
Bo had abstained for two days, saving his energy. Originally, he was supposed to leave today, but because he got nothing, he decided to stay one more night.
Plus, his wife shooting in a swimsuit would easily draw attention. Bo was very aware of how good Xia Siyu's figure was, seeing it every day. If any frivolous suitor came to leave their mark, it would be more trouble than it was worth.