Kaijuu No Kodomo

From Mangascreener: One summer vacation, Ruka meets two boys, "Umi" and "Sora," whose upbringing contains strange and wonderful secrets. Drawn to their beautiful swimming, almost more like flying, Ruka and the adults who know them are intertwined in a complex mesh... Meanwhile, an unexplained anomaly is occurring all over the world: fish are disappearing. Thus begins a marine adventure of boys and girls to captivate all the senses! Children of the Sea won the Excellence Prize in the Manga Division at Japan Media Arts Festival in 2009.