Isekai de Mizu no Dai Seirei Yattemasu.

異世界で水の大精霊やってます。

by Inaho Hodaka, Chihiro Aoki

When Saejima Nagi, a laid-back high school student, wakes up from his dozing off, it has become a lake in another world! Nagi learns that he is the "great spirit of water" with the lake as the main body, and heals those who suffer from injuries and diseases with the power that dwells in them. One day, Nagi is led by a call for help and is summoned to a boy. That meeting will greatly change Nagi's fate. (Source: Alphapolis translated)