Prior to our first cruise 20+ years ago my wife and I went out and bought two formal outfits each. I bought a tuxedo and a white dinner jacket, along with two different cummerbund sets. I still have them although they don't come close to fitting anymore.
I think we had the usual two formal nights on that cruise, along with two semi-formal, and I want to say there was at least one 'informal' night which still meant jacket and tie. Semi-formal was a suit. This was with HAL, and I think just about every man wore a tux or dinner jacket on the formal nights. Very few in suits.