Higyaku no Ningyo Hime

The story takes place in a world where humans and youkai coexist. Princess Yuki is a member of the prestigious and powerful Hime family, who possess the ability to transform into youkai. However, she is also a victim of a mysterious curse that causes her body to undergo a transformation into that of a mermaid when she comes into contact with water. As a result, she is isolated and unable to participate in the world above the surface. Yuki's only comfort is her connection to her loyal companion, a young boy named Taro, who accidentally releases the curse when they were children.