I'm not a laywer either, but after reading through a lot of these maritime laws and regulations in the last couple of days and taking in account that all these regulations are being made on the background of the International Convention SOLAS, I would say "Boundary Lines" are simply the country limits of the United States, that is a vessel is not undertaking international voyages. As soon as a ship is in international waters, SOLAS apply which makes it dispensable to have a national law for it as SOLAS is there already.