Andersen Douwa Ningyo Hime

Marina, the most beautiful mermaid in the royal Merman family, is curious about the human world. One day, she sees a handsome prince on a boat. Suddenly, a storm comes up, throwing him into the sea. Marina saves her handsome prince and guides him to the shore. Marina falls deeply in love and desperately wants to become a human. Despite the hazards, she is willing to risk it all for love.
(Source: Anime News Network)