I personally think that No.6 is located somewhere that China used to be. When Shion's discussing the Babylon Treaty in the anime, it shows ruins underwater, and at the mention of most of the world becoming inhabitable, my thought it that nuclear bombs were used in the last war, which could be powerful enough to change the climate itself. So I imagine that the process of global warming was sped up, a significant portion of coastal lands are now underwater, and basically all the islands we know of now are underwater as well. Plus, like we said, Nezumi's only been walking as far as we know.