Another lucky guy going on my favour cruise ship!
Don't know if you're asking for a man or a woman, or both. In general, "resort casual". No suits, jackets or ties. Nice shirts and pants for me in the dining rooms. Summer dresses and other resorty outfits for women. A wrap for when it's air-conditioned.
During the day, it's shorts and capris, bathing suits and cover-ups. Bermuda shorts, polo shirts, whatever. Ask away for specifics.