We did the Northern Lights cruise in February 2019. Don't overpack. We each took a down jacket, one pair of snowpants, fleece tops, long underwear, winter hiking boots, warm hat, gloves, socks, snow goggles, and a balaclava face covering. We also took "hot hands" hand and toe warmers and they worked great. It was actually warmer in Norway than it was here in Ohio.
FYI, if you plan to do any of the wolf excursions, we were advised (via a memo in our cabin) that you could not wear down jackets or leather belts. We opted to do the husky excursions (highly recommended!) which did not have those restrictions.
We enjoyed this cruise very much but, unfortunately, did not see the lights... If we were to do it again, we'd book a private tour so we could actually "chase" the lights and stay out longer. Hope you have a great trip!