Ningyo Hime Marina no Bouken

人魚姫 マリーナの冒険

Marina, a lonely mermaid, is bored of underwater life. She wants legs. She goes to the sea witch. She is given a magic whistle that when blown, can turn her into a human or mermaid. The Prince and Marina fall in love but she can't stay with him until she can get the sea witch off her case. (Source: Anime News Network)