Weathering With You/Tenki No
“Umi no Yurei” (“Ghosts of the Sea”) by Vocaloid/musician Kenshi Yonezu.
Studio 4°C is the anime house associated with the film Kaiju no Kodomo (Japanese Name). Children of the Sea Beasts is a manga that ran from 2006 to 2011. Viz Media published the manga in English, and it describes the story:
When Ruka was younger, she saw a ghost in the water at the aquarium where her dad works. Now she feels drawn toward the aquarium and the two mysterious boys she meets
there,Umi and Sora. They were raised by dugongs and hear the same strange calls from the sea as she does.
Ruka’s dad and the other adults who work at the aquarium are only distantly aware of what the children are experiencing as they get caught up in the mystery of the worldwide disappearance of the oceans’ fish.Viz Media
The main cast includes, Hiiro Ishibashi as Umi (left in the image below) and Mana Ashida as Ruka Azumi (center). Airu Kubozuka was initially announced to play Sora (right). However, he dropped out of the role since he had trouble with his voice changing due to puberty.