18+ Frogman

Michiru is a high school boy who isn't really good at anything, including sports and socializing. He likes watching one of his classmates, Haruka, as she practices swimming. Haruka happens to be the only member of the swimming club associated with his school. As she and the coach search for members, she notices him and innocently believes he's been watching her because he really wants to swim. Michiru is unable to say no to her invitation to the club, even though he's never swam before and has no idea how it's done. Michiru finds his life changing as Haruka patiently teaches him how to swim.

(Source: MangaHelpers)

Included one-shot:
Volume 1: Frogman (pilot)