Casual attire is the order of the day onboard. It's recommended to wear rubber-soled, flat shoes with good traction while walking around the ship, especially when out on the open decks.
However, there is a dress code at night. On cruise casual evenings, men are encouraged to wear slacks, khakis, jeans or long dress shorts and collared sport shirts. Casual dresses, skirts, pants, capris, dress shorts and jeans with tops are fine for the ladies. Not permitted in the main dining rooms on cruise casual nights (or on elegant nights, for that matter): flip-flops, bathing suit attire, cut-off jeans, sportswear, baseball hats or sleeveless shirts for men.
On cruise elegant nights (held once on sailings that are five nights or less in duration, and twice on sailings that are six nights or longer), men should wear dress slacks and shirts; sports coats, suits and tuxes are an option, though we didn't see a single tuxedo on our sailing. Women should pack cocktail dresses, dressy pantsuits or fine skirts; based on our cruise, it seemed as fine an excuse as any to get a second wear out of those old bridesmaid dresses. Note that children are also expected to adhere to the set dress codes.
Passengers who prefer casual attire at dinnertime, even on cruise elegant nights, can overlook the main dining rooms in favor of the Lido buffet restaurant, which has a more relaxed dress code throughout the cruise. The only real requirements are wearing a shirt, for those coming in from the pool area, and to wear shoes, though even flip-flops are OK here.
One of the recurring complaints we heard from Carnival loyalists was the failure of many passengers to adhere to a dress code on more formal nights. We saw several diners dressed inappropriately in sporty tops and ball caps at our early-seating dinner on one cruise elegant night. Officially, ship staff is supposed to ask passengers not adhering to the dress code to change before returning to the dining room.