Wear what you want to wear, formal night is mainly to have photos taken. You will still see other people dressed like they are at Ruby Tuesday, some in tuxes. I dressed up for dinner, then after dinner changed to go to the show, after dinner all the ships photographers pack it up and go develop the film.
You will be fine.. many wear basic black.. not all are in ties and gowns.. many are in simple pants and shirts.... very simple. Black is the color to wear, classy.. you will be fine.. dont' worry...keep it simple and .. all the best.. Gary