Pack like you would for any resort. Cruise vacations are casual by day, whether you're on the ship or ashore. In the evening, ships vary as to dress. As on shore, attire is dictated by occasion. For the Captain's Gala, for example, you'll probably want to wear something more formal, such as a dark suit, or cocktail dress; perhaps even a dinner jacket or gown. But don't buy a tuxedo just for the trip. If you plan to hike or go on a shore excursion, you may need a sweater and light jacket. You can find a complete cruise packing guide here.