We enjoyed getting dressed up on HAL, but those were also the days of cruislineformalwear.com which made it easy. I have a tux but not real fond of schlepping it on the airplane so leave it at home. Maybe someday... Right now, my wife wears a nice skirt and blouse, I wear black or navy cargo pants (that you can't really tell) and some kind of button up shirt (often a really nice Hawaiian shirt, think Nat Nast, Tommy Bahama, or the like).
Some of our favorite pics of us are on formal night in my tux and my wife wearing some of her nicer pieces of jewelry.