We were on the Marina for New Year’s Eve several years ago. Dinner in the MDR was delicious, with several holiday entrees offered. The midnight party was on the pool deck (we were in the South Pacific). Lovely colored lights, champagne, dancing, DJ and live music. Cardboard top hats and plastic tiaras for everyone. The entertainment staff mingled about, ensuring cheer for all. (No fireworks, however, as they would have been dangerous and environmentally naughty.)
Passenger dress was mostly country club casual, with a few tuxedos and evening dresses mixed in.