Probably not much help, but if you really plan to be out on deck, do take some real outerwear--the trench idea. I would rather be warm than stylishly exposed.
These cool weather "cruise" events can be a pain for dressing.
If there's truly a great indoor room and that's where people are congregating, you can be warm. Or sometimes you can step out for a few minutes off and on and retreat to warm up. Otherwise, I completely freeze on deck without pants and a jacket, and the whole beauty of the outdoors is lost on my shivering self, because we're not expected to dress for it . That is, going by your description--not a tropical cruise. Worse than cocktail parties with frigid AC, due to the breeze and falling night temps. So, sheer hose, long dress and dainty wrap would not do it for me. Of course, if I had unlimited semi-formal wear I'd wear a black velvet cloak or glitzy trench or something to fit the occasion.
Hope you can find the right outfit, but the outerwear can be the fallback for warmth. If you did find a great dressy pantsuit that would work too, I think--I've done that to wear warm underthings.
You may not be as bothered, or maybe it won't be that cold, so take that with a grain of salt.
