I've only been on one cruise, and it was another line. But the truth is no one gives a hoot what anyone else is wearing.
People like me who went out and sprung for a tux (I built my own out of a nice black suit) might get all uppity. But its because you make us look over dressed. But here's the key ... you will definately NOT look under dressed, unless you show up looking like a slob in a tee shirt and gardening pants. And believe me, some people do that too. I suspect those are folks who won the cruise. So who can blame them.