Beatrice, when I was on Maasdam last fall, formal ranged from "Sunday dress" (nice but not formal dresses or pantsuits) to sequins for women, and from sport coat & tie to tuxes for men. You could dress up an otherwise "business casual" pantsuit with sparkly accessories too - a costume-jewelry shop has them for not much money.
If you have time & budget for it, I recommend getting a nice dressy skirt - perhaps a chiffon or gauze - of mid-calf length, and a glittery top or two to go with it. That has worked well for me. (Skirts with elastic waists are even better). Appleseeds catalogues often have something like that, and now that fall fashions are in the stores, you might find something at your favorite dept store, perhaps even Walmart! There will be 2 formal nights on your 7-night cruise.
Have a great time!