I was at the Pensacola Air Museum several years ago and saw a display of a plane that went down , I believe, in one of the Great Lakes. It had been under water for decades. Part of the display was the Browning machineguns that had been on board. Other than some very light surface pitting, the guns were functional and visually appealing. The water was of course was fresh, deep and cold. I don't think the ocean would be quite as forgiving.