TBH it depends on how cold-natured you are. Unless we're talking below 70 or so, I'm fine without anything - but there were people bundled up like they were in Alaska on Pirate Night a couple of weeks ago which was I believe a little over 70.
We've sailed in January, and we were fine on deck for the pirate party. What we needed light sweaters for are all the MDRs and theatres, so pack something for sure. Also, one of our sail away parties was a little chilly.
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