Mermaid Melody

Lucia, Mermaid Princess of the North Pacific Ocean, sets out to land to find the boy she saved from a shipwreck seven years before the beginning of the story, the boy to whom she entrusted her pink pearl. She eventually found the boy: a surfer named Kaito Dōmoto. But as the human form Lucia assumed, a girl named Lucia Nanami, bears very little resemblance to her mermaid form, Kaito cannot recognize her. She cannot tell Kaito who she really is; otherwise, according to mermaid folklore, she would turn into foam. So she tries to convince Kaito into figuring out who she really is (since the legend does not say anything about the other person discovering a mermaid’s true identity by himself).

At the same time, Lucia has been told that a group of water demons or suiyō (水妖) have invaded the marine world and she must gather the six other Mermaid Princesses and their pearls to resurrect the legendary goddess Aqua Regina to stop them. To fulfill this, she joins forces with Hanon and Rina, Mermaid Princesses who came up to dry land, use their pearls, turn into singing idols, and use their voices as an offensive.

-wikipedia