If you're goal is packing light, consider items that do double or even triple duty. For example, I have a great black pareo (a large cotton square with fringes) which looks good as a bathing suit coverup (my bathing suit is black and white), but also looks very suitable worn as a skirt (make sure it's big enough to wrap so that it doesn't open too much when you sit down!) with a fresh white t-shirt and black sandals, and also serves as an attractive shawl in the evening over a sundress to wear inside (restaurants, movies etc.) I take comfy linen pants which work really well with a tshirt for being in town but also dress up nicely at night with a silk blouse or sweater. I have my favorite pair of sandals which are pretty enough to get away with at night but comfy enough for a day's walking - mine are from Easy Spirit and I love them! Big XXXL tshirts (like a multi pack from Hanes) are great for sleeping in and can also do for a swimsuit coverup. They can be decorated for special events (talent show? Tropical night?) if you're creative (bring fabric pens, for example) or signed by characters as a souvenir, perhaps? Or, they are so inexpensive, they could be discarded at the end of the cruise and make a little extra room for souvenirs!
In addition to multi-duty clothes, try picking (and sticking with) a color scheme so you can mix and match easily. For a cruise, blue and white or black and white or red and white (or a combo) looks very fresh and very nautical. I try to make a mainstay of a summer trip wardrobe a good quality t-shirt (Brooks Bros. has nice ones) in a good neutral color (white, my favorite!) which is neat and tidy, easy to wash (or bring several!) and a good background for accessories, which makes wearing the same thing look totally different (think scarf!)
I also pack few bottoms and lots of tops, so I can be fresh without lots of extra outfits, and plenty of undies and accessories, which don't take up much room. This way, you can even avoid doing laundry on holiday. Of course, if you're willing to do laundry, you can get away with even less.
Hope that's useful.
Have a great vacation!
Karen