We did Norway/Iceland several years ago. It was in June so earlier in the year than your cruise. My standard sea day outfit (on any cruise) is usually a tee-shirt, capri's and a sweater. I think maybe I switched to leggings/yoga pants for the North Atlantic part of that trip, but it wasn't abnormally cold. And I have pictures of people swimming on a sunny day, but I also remember more grey gloomy days.
My son got an ear flap hat in Norway that he ended up wearing during the Iceland part of the trip, but really it was only cold in one spot for one part of the day. The rest of the times I'd say it was chilly, and we needed raincoats/windbreakers, but never thick coats or anything like that.
The whole trip was warmer than the cruise when we left NYC in November (the seas weren't as bad either).
