Isekai de Mizu no Dai Seirei Yattemasu

When the laid-back high school student Saejima Nagi wakes up from a nap, he finds himself in a lake in another world!? After learning that he is the "Great Water Spirit" whose essence is the lake, Nagi uses the power that resides within him to heal those suffering from injuries and illnesses. One day, Nagi is led by a voice calling for help and is summoned by a boy. This encounter will greatly change Nagi's fate...