Unlike other major exhibitions of the art deco period, DecoDence: Legendary Interiors and Illustrious Travelers Aboard the SS Normandie, which opens today at the South Street Seaport Museum, isn’t an over-the-top display. Instead, it’s a balanced, and entirely engrossing, collection of furnishings, ephemera, and architectural elements that graced the legendary ocean liner. Among the show’s highlights: photographs that document the …