Mermaid Melody - "Pichi Pichi Pitch - aqua"

Lukia Nanami is turning seventeen. Her family has just moved to a town by the sea, and on her first walk down to the beach, she saves a handsome surfer from drowning. The next day, she's shocked to discover he's not only a classmate at her new school, but has no idea who she is, and insists he was saved by a mermaid! (source: Kodansha)