Pichi Pichi Pitch: Mermaid Melody

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)

Included one-shots:
Volume 7: Moonlight Goddess Diana, Get Nude, Cherry Blossom