Perhaps in one of Sun's antics out in the water, like the female swimmer teaching him how to swim or hanging out with a few guys, he could make the female swimmer pee in the water via surprise finger shove in her asshole, making the others recognize the sign of...warmer water and them fleeing, all but Sun.
I also liked detailed groping/stimulation, so if you could indulge on where he grabs or touches that can help too.
If they do get intimate at the end of the story, it should only be fitting that one sex scene has him fingering or messing with her clitoris so that he can 'milk' out her bladder good, before fucking her (yet she still pees midcoitus/climax)