Interesting, I was wondering the same thing. We're going on a Christmas cruise in the Caribbean that time of year. Since the velvet I have is also light weight, but long-sleeved and to the floor, I'm going to pack it for formal night, but bring a warmer weather cocktail dress, just in case I change my mind.
Our last cruise found ladies and gents in everything from wedding gowns and tuxedos, prom dresses and gloves, to capri pants and tank tops, and one Wyoming cowboy in his wranglers, boots and sporting a wife beater tank top. Wear what you want! If you have a velvet Christmas gown, enjoy wearing it! The dining rooms are definately cool, so you'll be fine. Since velvet does crush, trust the laundry service to press it the day before you need to wear it. Have fun!