We did a cruise of the Norwegian fjords years ago. There were a couple of days of our 10 day cruise that were full days at sea in the North Sea. There was one day that was particularly rough and quite scary. I agree with another poster above that the North Sea can be unpredictable. If your cruise goes all the way to Nordkapp (as ours did) you will have at least a full day at sea to get there. The waters within the fjords were relatively calm. We enjoyed the trip but wouldn’t travel there again. Even in mid summer the weather was kind of miserable, but the scenery was breathtaking.