Shipboard dress is casual during the day. Shorts and tasteful T-shirts are acceptable even in the dining room. At dinner, dress codes will vary daily, among the following:
Casual: Slacks and sport shirt for men (no jeans); slacks and blouse, sundress or pantsuit for women.
Informal: Jacket with or without tie for men; dress or pantsuit for women.
Formal: Dark suit or tuxedo for men; evening gown or cocktail dress for women.
Most 7-night cruises have 2 formal nights; most 10-night cruises have 3 formal nights, and so on. In general, the longer and more expensive (per day) cruises will be the most formal on formal night, but a dark suit and tie is acceptable attire on even the most exclusive ships. (In other words, don't buy a tuxedo for a single cruise).