Check with your specific cruise line to see the dinner dress requirements. Many have formal dining on some evenings so you'll want to see if that applies to your vacation. Cruise lines have various definitions of how formal they want their guests to dress; some require tuxedos for the men and evening gowns for the women. Some ships with formal dining requirements will turn passengers away from the dining room if they're not wearing proper attire. Most ships with a formalwear requirement will rent or sell tuxedos and gowns onboard. Even if formal dining isn't a requirement, you'll want to pack something that's dressy: suits for men, and dresses, skirts, and blouses or a dress suit for women.
Daywear is more casual. Shorts and T-shirts are usually fine for warmer weather, and jeans will come in handy. Remember that weather at sea can change quickly, so be prepared to dress in layers. A light jacket is a good idea, even in the summer. You might want to also pack a light raincoat. Bring sturdy, broken-in walking shoes for shore excursions; with trips to Italy, Spain, Monte Carlo, Corsica, and other places, you'll be doing a lot of walking. Bring a hat--you'll want it for shore excursions and for daytime outside on the ship.
When it comes to swimwear, use discretion, especially if it's a family cruise. If you plan to spend a lot of time by the pool, pack at least two bathing suits. Bring a cover-up as well--many cruise lines will allow you to access the daytime buffets wearing a bathing suit if you're wearing a cover over it. Don't forget flip-flops or water shoes for the pool area. Bring workout clothes if you're planning on using the ship's fitness center.
