Mizu Zokusei no Mahoutsukai

Ryou is thrilled to be reincarnated in Phi, a world where he intends to live a tranquil life mastering water magic. However, his peaceful aspirations are immediately challenged by the dangerous subcontinent he finds himself in, teeming with deadly monsters. Despite the constant threat to his life, Ryou remains optimistic and clever, and his hidden "Eternal Youth" trait allows him to rapidly gain experience. Over twenty years, he hones his skills, striving to become the pinnacle of human magic. His journey, however, is just beginning. A significant encounter soon propels him into the heart of history, forever altering the course of his life and setting the stage for his adventures as the world's strongest water magician, who still prefers to take things at his own pace.