Sirenetta: Part-Time Mermaid (1970)

Nettie, a modern Little Mermaid, is living humanly-ever-after in Provincetown with her fiancé, Freckles. When an anxious young merperson named Archie and Calypso the Sea Nymph come looking for help though, the part-time mermaid dives back into action. Together, Nettie and her three friends travel the world in search of the Terzetto Treasure; a magical artifact that will stop an evil sea witch and her coven from draining the ocean of life.