Check with the Maitre'd when coming aboard to confirm arrangements. Most cruises have at least one formal dinner, show or on-board event when you wear a tuxedo, suit or gown. Of course sun lovers will need to bring a bathing suit. For the Caribbean, more cotton, breathable materials, and etc. for hot and humid days. On any cruise, don't forget your tennis shoes for walking the deck, aerobics and working out, and appropriate footgear for shore excursions, dancing, etc.
Quality is a relative term, yet in the cruise industry, it is the norm, due to competition.
Itineraries are also varied, with the majority being 7 night packages. Bargain hunters can sometimes find good value by taking a cruise when the ship is relocating from one seasonal area to another, many times through the historic Panama Canal. As always, you get what you pay for. A newer ship with more room, more amenities, etc. is going to cost more. The rates are generally based on double occupancy, per person, but be sure to ask if the room is actually a double room or if it is designed to accommodate 3 - 4 passengers.