Saban's Adventures of the Little Mermaid(1991)

Ningyo Hime Marina no Bouken

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)