Isekai de Mizu no Daiseirei Yattemasu.

Nagi Saejima, a carefree high school student, wakes up from a nap and finds he has become a lake in a different world!? Nagi learns that he is the "Great Water Spirit" whose main body is the lake, and he uses his power to heal those who suffer from injuries and diseases. Eventually, led by a voice calling for help, Nagi is summoned by a boy. That encounter will greatly change Nagi's fate... (Source: AlphaPolis, translated)