Mermaid Melody Pichi Pichi Pitch

Based on a manga of the same name, Mermaid Melody Pichi Pichi Pitch is a magical girl anime starring mermaids, and specifically the mermaid princesses of the seven seas, who battle their enemies (so to speak) by singing.

Lucia, princess of the North Pacific Ocean, is the main character of the show, and following familiar beats of the Hans Christian Andersen story, she once saved the life of a human boy when he fell into the ocean, and not too long later she travels onto land in the hopes of finding him and getting him to return her love. Lucia's best friends Hanon and Rina are also mermaid princesses, and together the trio magically transform into idols and use their music to save the day.

There is a whole host of other mermaids and merfolk through the series, though my understanding (I haven't watched the show in full) is that most of the story takes place on land, though there are plenty of underwater scenes and worldbuilding since most of the princesses' enemies are water-based folk.