In my opinion there really is no difference between casual and smart casual. Everyone pretty much wears what they want except for the formal nights when everyone is really dressed up. As long as you are not wearing jeans or shorts you will most likely fit in on casual or smart casual. Don't stress over your outfits for those nights. I have seen everything on nights when it was not formal. Even when people do wear shorts no one asks them to leave. But you are on a cruise and that is a time to dress up and "go to dinner" so as was said before nice slacks and a polo shirt will be fine for either casual or smart casual, ladies can get away with almost any type of nice outfit with sandals or dressy flip flops. Don't stress over your outfits, I always take too much and end up with 2-3 outfits that were never worn to dinner!!!!