Kappa no Sanpei

A boy named Sanpei lives with his grandfather in the mountains. While napping on a river raft, he is mistaken for a Kappa and taken underwater to their kingdom. Mistaken as a human, he is sentenced to death. He argues that humans have advanced knowledge the Kappa could benefit from, persuading the elders to spare his life. As a precaution, his bellybutton is removed, believed to house his soul, before he is returned to the human world. He is accompanied by a Kappa exchange student who looks very much like him.