Without limiting the power and authority otherwise vested in the Federal Maritime Commission and the Secretary of Transportation, it shall be unlawful for any common carrier by water, either directly or indirectly, through the medium of an agreement, conference, association, understanding, or otherwise, to prevent or attempt to prevent any other such carrier from serving any port designed for the accommodation of ocean-going vessels located on any improvement project authorized by the Congress or through it by any other agency of the Federal Government, lying within the continental limits of the United States, at the same rates which it charges at the nearest port already regularly served by it.