From MangaHelpers:
The water polo team at Kamakurahama High School has a new ace player, Torii Ushio, who you wouldn’t expect to be a star athlete if you knew her in the classroom. Nakaoka Riku always took her as a shy person but after seeing what she can do in the water and how fast she can swim, he couldn’t deny that she deserved to lead the team. But late one day after practice, Riku finds out that Ushio has some secrets she has been keeping from everyone. Her family runs a temple and Ushio is the priestess to the dragon god. The descendent of the original dragon god is Sango who is a teenage girl who is the same age as Ushio. She lives in a secret underwater cave and her only friend is Ushio whose role is protector and companion. Through a series of events, Sango joins the water polo team to be close to Ushio and amazes the team with her out of this world swimming ability.