On a recent Carnival Cruise I observed the following on "Elegant Night":
Men: about 35% still wearing a suit with tie, about another 35% wearing either a dress shirt and tie, or a shirtw/tie or shirt/no tie/jacket, and the remaining 30% just with a collared shirt
Women: About 40% wearing evening gowns/prom dresses, 40% dressy cocktail wear, and the remaining 20% wore whatever.
With most airlines charging now for luggage, I suspect more and more cruisers will leave the formal wear at home.
The following is a cut and paste from Carnival's web site regarding suggested dress for Elegant Night:
Cruise Elegant Dining Dress Code: Gentlemen - Dress slacks, dress shirts. We also suggest a sport coat. If you wish to wear suits and ties or tuxedos, by all means we invite you to do so. Ladies - Cocktail dresses, pantsuits, elegant skirts and blouses; if you‘d like to show off your evening gowns, that's great too! Not permitted in the dining room during the Cruise Elegant dinner for ladies and gentlemen: shorts, T-shirts, beach flip-flops, bathing suit attire, jeans, cut-off jeans, sleeveless shirts for men, sportswear, and baseball hats.