Isekai de Mizu no Dai Seirei Yattemasu.

High school student Nagi Saejima wakes in another world transformed into a lake and discovers that he is the Great Spirit of Water. With the lake as his main body, Nagi uses his power to heal the sick and injured. One day, a boy calls to him, and Nagi follows a voice asking for help. Their encounter sets Nagi on a drastically changed path.