Remain (2021), Japanese Anime TV Series

Plot:

In the winter of the 3rd year of junior high school, Minato Shimizu, who won the national water polo tournament, was in a coma due to a traffic accident on his way home.

Although he finally worke up 203 days later, he had lost his memory of his 3rd year of middle school.

It was Minami who left the water polo and newly entered Yamagata High School, but with one promise, he started water polo with friends he met there.

However, various difficulties await in the weak water polo club…

Reference: Remain Official website