Mermaid Melody: Pichi Pichi Pitch

Lucia is the new girl at school. She and her sister run a public bath that's all the rage. When Lucia meets a terrific-looking surfer boy, there's just one little problem: Lucia is a mermaid-not just any mermaid, but a princess on an important mission to save the seven seas from an evil force bent on taking control of the marine world. Such a responsibility doesn't leave much time for romance. But Lucia vows to protect her world and win the heart of handsome Kaito.
(Source: Del Rey)
Note: Includes the one-shots Moonlight Goddess Diana (MOONLIGHT GODDESS ディアナ), Nude ni Natte (ヌードになって, Get Nude) and Cherry Blossom (チェリー ブロッサム), contained within Volume 7 of the manga.