The One Piece world's most distinguishing feature is its geography. Its only continent is a strip of land encircling the globe called the Red Line. The rest of the world is one large archipelago comprised exclusively of islands. Few fans questioned this layout because it made sense for a pirate-themed manga. It was the ideal setting for Monkey D. Luffy and the Straw Hat Pirates to sail their ship from one destination to the next. There was little reason to wonder about the in-universe reason why this world is covered in so much more water than the real world.
Source：https://www.cbr.com/one-piece-theory-there-were-once-continents/
