Umisho

From ANN:
Kaname Okiura, a student of Prefectoral Umineko Shougyou High ("Umisho" for short), joined the school's swimming club in order to learn how to swim, but the club is filled with weirdos, let alone teaches him swimming. Then, a sunny, happy-go-lucky girl named Amuro Ninagawa who was transferred from Okinawa joined the club. Her extraordinary underwater speed and unorthodox swimming style (plus her personal habit of nude swimming—a powerful magnet to teenage boys) surprised every club member, especially Okiura, because she reminds him of a mermaid-like creature he saw only once in his early childhood.