Before you pack any type of costume, i would ask your travel agent or the cruise line if they have some sort of Masquerade Party on Halloween or if its strictly formal night--that could be a blast if they did something for Halloween - You could go dressed as a Princess, Cinderella, Mortica Aadams, Elvira, Lady in Red, or if you didn't want to go in full costume, wear a long dress or evening gown and a Mardi Gras type mask
We cruised last year on Halloween (and actually will be this year also). I wore a blask feather boa and carried an eye mask with feathers (on a stick). It worked well. Most people were dressed for formal night, as was I (with the additions). My husband stuck to the tux.
My other suggestion & I was actually thinking of this myself (but alas now I can't go on the Halloween cruise) was a beautiful flapper dress. I bought mine as a costume but it's really elegant with a drop waist & a rhinestone knot where the bow sits below my hip. Lots of pearls & a tiara.