Quote from jackophantRegarding the size of the map, in fallout 3 a lot of DC wasn't traversable. You kept taking loading screens between places. FO4 has much less of that and is far more free roaming. I also refuse to believe that there is nothing in the right portion of the map which is mainly sea... There must be something down there, maybe Rivet city will somehow be operational and float along?
Don't forget there's a perk which allows you to breathe underwater!