The Hermit Crab and the Straw Hat

When Kenji Tomosaka moved to a new coastal town, many years ago, he met a girl on the beach who is playing with some hermit crabs. She runs away crying after Kenji puts a crab on her head. When he gets back to his new home he finds out the girl is his next door neighbor and that his and her balcony are close enough he could jump from one to the other.

Synopsis excerpt via Kitsu.