Akari no Moribito: Seiden
The year is 2045. A world in which the coastline is largely submerged. A young man named Yukari Fujikura is attacked by a sea monster known as “Suika” while on a quest to find his missing grandfather. Yukari is saved by the "Guardian of the Light," an incarnation of the light that resides in a lighthouse. After joining the “Pilgrimage Agency,” an organization that fights against the Suika, Yukari sets out on a journey around the lighthouses with his superiors, Sen Nakazawa and Akiko Douke. Their ultimate goal is to save the future of the world…
(Source: WONDER WAVE, translated)