Aqua Puella: Shikinomori Koukou Suieibu

The manga centers on Konta, once a promising athlete in competitive swimming. In a past beach trip, he tried to save a girl from drowning. Though his efforts saved the girl's life, the girl remained comatose. Feeling guilty for his own perceived failure, Konta stepped away from swimming. Five years later, Konta has become a shut-in and a shadow of his former self. Unable to stand what has become to him, Konta's older sister suggests he become a swim coach to Shikinomori, a high school known for its competitive swim team. Though not really motivated, Konta heads to the job, and there he finds the girl he saved five years ago.

(Source: Anime News Network)