In the past, evening meals were black tie, but as we have become so much more casual in our business lives it seems the cruises have relaxed there "Formal" criteria to be pretty much coat with a collared shirt and a tie may be optional. It dependes on the cruise line but the age of mandatory tuxedo's are gone.
More's the pity, really. I wear my formal attire any chance I can get. Page & I are planning to take a cruise someday, and my kilt and prince charlie with me for evenings.

Cheers,

Todd