Ningyo Hime no Himitsu

Lise the mermaid has a secret treasure among her companions. It is a painting of a beautiful young man that was on board a sunken ship. She breaks the mermaid tribe's rule of not getting close to humans and frequents the seaside castle, where she finds small happiness in gazing at the prince, who resembles the painting, from afar. She has had unrequited love for the prince ever since he saved her as a child. With her feelings for the prince and her interest in the human world growing, one day Lise decides to enlist the help of a witch to become human, but...

(Source: Hakusensha, translated)