Aoi Kiseki

From Nagareboshi Manga:

All the girls are crazy over the swimming prodigy Wakatsuki Ran. Ageha, who wants to find love but isn't really looking that hard, bumps into him one day, and after surviving his sarcasm instantly pegs him as a "stupid, obsessed swimmer". Nonetheless, she is amazed by his talent and asks him to teach her to swim like him. How will their relationship develop?