More casual. We saw mostly nice Sunday clothes on formal nights, and just nice slacks sets for other nights. Not in the dining room, but everywhere else, we saw jeans and jogging suits in the public areas.
a lot also depends upon which line you are sailing on. Some are more formal, some less. If you are concerned, you can't go wrong with a nice long dark skirt and a dressy/sparkly top with heels, or your basic little black cocktail dress is always proper.
Dark suits, dress shirt & tie or tux for men.