Big Fish & Begonia

In a mystical realm that exists beneath the human world, populated by magical-powered beings, a girl named Chun participates in a coming-of-age ritual where she is transported through a portal of water to experience the human world in the form of a red dolphin. There, she encounters a human boy who lives by the sea and reveres aquatic creatures. During a storm, Chun is tangled in a fishing net near the boy's house, and the boy drowns while freeing her from the net. Chun returns to her world, taking the boy's ocarina with her. Read more on Wikipedia.