Here's my experience on "formal" night.
On our 11-night last year, we had two formal nights due to the length of the cruise. Both nights, I saw men in everything from tuxedos to -- I kid you not -- blue jeans and a short sleeved shirt, worn with athletic shoes. That man's wife was in a nice dress
I saw men in suits, I saw men in dress shirts but no jacket or tie. I saw men in a dress shirt and tie, but no jacket. In other words, the whole nine yards.
For women, I saw women in gowns, yes, but I also saw women in dress slacks and nice tops, skirts and blouses, and sundresses with a wrap over their shoulders. Or, no wrap.
I saw heels, I saw flats, I saw sandals. Me, I wore dressy sandals. And a nice dress with a wrap over my shoulders, but nowhere near what you'd call a gown.
My personal opinion is to go as dressy as you're comfortable with. If you're uncomfortable, you won't have a good evening no matter how nice you might look