No oil or oily mixture, hazardous chemical or obnoxious substance from a mechanically propelled vessel shall be discharged in inland water:
     Provided that nothing in this section shall apply to the discharge of such oil or oily mixture, hazardous chemical or obnoxious substance from a mechanically propelled vessel for the purpose of securing the safety of a mechanically propelled vessel, preventing damage to a mechanically propelled vessel, cargo or saving of life at inland water.