Suiei Kyoushitsu ~Fuyu de mo School Mizugi~

The protagonist, Touya, is a member of the baseball club. After returning home from club activities, he realizes that he has forgotten something important: he has forgotten his step-sister Naho's birthday and has not prepared a present for her. Naho cries and protests, "You're so stupid, stupid, stupid, stupid, big brother!" Touya apologizes, saying that whatever the reason, it was wrong to forget his stepsister's birthday, and instead of a birthday present, he tells her, "Please forgive me, I'll grant you one wish, Naho."

Naho's wish was, "Take me to the beach next year. I want to swim with you, big brother." Naho had often invited Touya to go to the beach up until now, but Touya had always refused. This was because Touya couldn't swim (he was terrible at swimming). However, Naho never dreamed that Touya, who had excellent athletic ability, was terrible at swimming. Touya has promised Naho that they will go to the beach next summer, and thinking that this will be a good opportunity to overcome his fear of swimming, he decides to attend a swimming school in the next town so that no one will know that he is bad at swimming...

[From Getchu]