§ 56. Fire pump. Every public vessel permitted by her certificate to carry one hundred passengers or more, shall be provided with a fire pump or other equivalent apparatus for throwing water, the same to be at all times during the navigation of such vessel, kept ready for immediate use, having hose of suitable size and sufficient strength to stand a pressure of at least seventy-five pounds to the square inch, and of a length to be specified by the inspector. The inspector may require the installation of an approved fixed fire extinguishing system as defined in subdivision five of section sixty-seven in lieu of, or in addition to, the fire pump required by this section.