Taiikukan Baby (2008)

They're two versions of the same story (you can think of "Classmates" as a touching side story).
In "Classmates" Jun meets a girl through e-mail and falls in love with her; in "Taiikukan Baby" Jun is courted by his rival and classmate (male).

While both deal with swimming, 'Taiikukan Baby' has romance undertones where 'Dansui!' does not. Both are movies focused around swimming and the struggles swimmers face.