I guess in the end it means you should dress in good taste. If you are on the ship in the public areas on formal night, I don't think that should apply to the show or to the library - maybe the dining areas, lobby/atrium and lounges only.

If I am in a simple black dress, who can classify that as casual, informal, formal?

For men in a dark suit, isn't that formal enough? My husband wore a suit every day for 30 years to work and will wear them for a special occasion but prefers not to wear a coat and tie unless we are doing something like theater.
In California it is rare to see a tie these days unless you are an attorney going to court. I see kids in Jeans and shorts in the best places these days - everywhere. It was a sign of respect to dress appropriate to the occasion.

I don't think that evening wear needs to be a tux, but, I do like to see everyone dressed to the NINES now and again.