For the many cruises taken, I have always brought along a plain black suit. I pack a formal shirt with studs and a black bow tie. I also bring along regular business shirts with standard ties.
The decision to go "formal" or not is usually taken after you have met people you will be either dinning with or some social activity where people make their position clear on formal dress. If the table indicates they are going formal I will don the black suit and formal shirt, etc. If they decide to go business, I do likewise with regular shirt. The black suit is almost indistinguishable from a tux especially when socializing in such a large crowd.
For the sake of packing one additional shirt and tie, having this flexibility has been most rewarding.