Children of the Sea is a difficult film to understand. Directed by Ayumu Watanabe and adapted from Daisuke Igarashi's five-volume manga, the film is philosophically comparable to Neon Genesis Evangelion. Watanabe's psychedelic antics have successfully intrigued, dazzled, and ultimately bamboozled countless viewers, some of whom still aren't clear about the movie's conclusion.
Source：https://www.cbr.com/children-of-the-sea-anime-movie-ending-explained/
