The Water Magician

Ryou dies in his late twenties and reincarnates into the fantasy continent of Phi with a brief: the magical specialization is water, the longevity is unlimited, the rest is up to him. He arrives in the southernmost wilderness, an uninhabited subcontinent populated entirely by dangerous monsters. The series picks up his first twenty years there alone: living off the land, working out water-magic combat applications none of the continent's known mages have tried, and being undisturbed by human politics for two full decades. By the time he is finally found by another human, he is one of the strongest mages alive and is mostly inconvenienced by it.