Uncharted Waters are the areas on the edge of the sea map and beyond. You can find it by sailing all the way into a horizon with no distant island showing, and eventually once you have gone past it, you cannot go any farther. The most dangerous and intimidating vessels sail in these waters; occasionally, one may even sight the rare and mighty Juggernauts. When the Treasure Fleets reach their destination, it is somewhere in Uncharted Waters.
Edge of the World
If you sail long enough, you will reach the "edge of the world." You will be told to turn around and continue your adventure. You cannot sail past the edge of the world.