You mentioned Carnival. If that is the cruise line you are on, or NCL, then you really don't need to go dressed to the nines at all. They don't even call it formal night anymore. Not that jeans and tshirt would be okay, because that would not be appropriate, but anything nice is fine. I usually take a pair of black pants, some solid color tank tops and 2 semi sheer over blouses. Add a pair of sparkly earrings and I'm good to go.
Not going to comment on the high end cruise lines because I know nothing about them!