My wife has a variety of outfits that are perfectly acceptable for formal evenings on board. They range from long dresses to an outfit that I call her RockStar outfit, which is something like a silky pair of pants with a bodice top, except it's one piece. It's a real pain for her to wear, but she looks so good in it that she wears it just for me.
Don't get too stressed about dress. If you have doubts that an outfit doesn't quite fit the bill, just accessorize.
9 Princess cruises to date
Sea, Dawn, Grand, Golden, Caribbean, Royal