Aqua Puella: Shikinomori Koukou Suiei-bu

Konta, once a promising swimmer, saved a young girl from drowning in the sea. Though she survived, she fell into a coma. Consumed by guilt, Konta abandoned the sport entirely.

Five years later, he has become a recluse, a shadow of his former self. Worried for his future, his older sister invites him to coach the powerhouse swimming team at Shikinomori High School. Reluctantly, he heads to the pool, and there he meets the very girl he had saved many years ago...

(Source: Kadokawa, translated)