Isekai de Mizu no Dai Seirei Yattemasu.

Saejima Nagi is your average, lethargic high school student, more accustomed to the comfort of a desk nap than any grand adventure. His quiet, uneventful life takes an abrupt and bewildering turn when he awakens from a doze to find himself not in his classroom, but as the living, breathing consciousness of a vast, tranquil lake in a world of swords and sorcery. He soon discovers that his new existence is that of the "Great Spirit of Water," his very being intertwined with the lake's crystalline depths. As he grapples with his extraordinary predicament, Nagi realizes that his newfound form pulsates with a potent, ancient power—a power that can mend wounds, cure ailments, and soothe the suffering of the inhabitants of this new realm. Embracing his role as a silent guardian, Nagi uses his healing abilities to quietly tend to the injured and diseased who come seeking the lake's legendary blessings. His days fall into a peaceful, if isolated, rhythm as he dutifully watches over his aquatic domain. However, this fragile serenity is shattered when a desperate plea for help cuts through his consciousness. Answering the call, Nagi follows its origin to a young boy, a summoning that marks a profound turning point. This fateful encounter will not only test the limits of his power but will also drag the reclusive spirit from his placid sanctuary into the turbulent currents of a conflict far greater than he ever imagined, forever altering the course of his existence.