Since you will be on a cruise if you are wearing something semi formal you will atleast have reason to wear it again on one of your informal nights. It will actually save room in your luggage by wearing it onto the ship.
What time is the ceremony? I saw that you get to board extra early but I am assuming you need to arrive in the clothes you will be wearing since rooms probably won't all be available.
It sure sounds like a fun way to start a cruise