Depends on where you are cruising of course. My experience in the Southern Caribbean is that it doesn't get cool at all on deck at night. You're still fine in shorts and short sleeve tops. A funny story is that on our first cruise ever out of San Juan, after dinner I went back to the cabin and changed into jeans and put on a light jacket, while my husband told me I was nuts. But I insisted the night air at sea would be cool. We walked out the automatic doors to the pool deck and whoosh.....the warm sea air hit me like a ton of bricks! It was one of the rare occasions that I told my hubby he was right (lol) and went back to the cabin to change into shorts and a t-shirt.
Eastern Caribbean in winter can be cool at night however.