Farewell, Ultramarin

Gun, a boy who has lived alone with his father on a deserted island, is told, "Experience the human world. The world you are about to enter is made up of humans." Following his father's dying wish, Gun is shipwrecked on an island with his father's body, where he encounters "humans" and falls in love with a young female diver. Gun is full of curiosity about the world he is seeing for the first time. However, the closed-minded islanders shun him, seeing him as an "ominous sign"... A straightforward and heartwarming coming-of-age drama that depicts the beauty of humanity and nature through the eyes of an innocent boy.

Source: MangaHot, translated