Boku to Umi Kanojo

It's not like I don't belong here... One year ago, a young boy named Mikane moved back to a small island in Kyushu. The locals are warm, his father is brusque yet kind, and he has a good friend who worries for him. Despite having so much, Mikane feels dissatisfied and lonely. Then, one day, he saves a mysterious, beautiful woman from the sea. His meeting with this "oneesan" will drastically change his destiny!!

(Source: MANGA Plus)