Everyone should assume that all food on a cruise ship, MDR, Buffet or Specialty, has salt added to it. Even the oatmeal at the breakfast buffet has been prepared with a "dash" of salt. (otherwise it will be inedible to most palates). You have to be super diligent and know what your actual tolerance is. Asking for low sodium foods is not realistic. Asking for no salt added while my food is prepared IMO is a better choice.
And sorry, telling the Head Waiter that you want low sodium food and then ordering a bacon cheeseburger is just giving mixed messages to the crew and not beneficial to anyone.