On a one-week big ship cruise, expect to dress up twice if you want to eat in the main dining room. Kids who can't be cajoled out of jeans or shorts eat upstairs at the buffets, and can enjoy dining al fresco if they wish. Here's the truth. Men can take one dark suit, a few dress shirts, and a choice of ties. I've seen tuxes, but they are rare.
Women usually like to dress up.
Cocktail dresses or nice pant suits suffice. No one wears ball gowns. A cheap way out is to buy one long black skirt, and wear it each night with a different blouse and jewelry. Although I have never had anything stolen from me on a cruise, I take only costume jewelry. Most days, aboard ship or off on tours, I wear sneakers, socks, shorts, a T-shirt and a brimmed hat.