I know that I will get my virtual hand slapped for this, but dark or light slacks should be fine. Some people will say that a sport coat is not formal enough, but unless you are going on one of the higher end cruise lines, don't worry about it. My hubby is a musician. He does not own a tux or a dark business suit, which is technically what cruise formal means on most lines. He resents the idea of spending money on clothing just to satisfy a few people who want the world to be their way. We are not rebelling or indifferent to social etiquette, but money is hard to come by for a musician (as well as many other professions), so I don't push the issue.