The weather in western Norway is surprisingly mild for much of the year - much warmer than places the same distance north in the US or Canada. We've recently come back from a two week cruise in June, getting right up past the northern tip of mainland Norway. The only time ashore that it was cold enough to need a jacket was at North Cape, and that was because it's 1,000 feet up on a very exposed plateau. Even on the few days ashore that it rained the rain didn't have the malevolent chill that it does back home. Going as near to midsummer as possible also means it never gets properly dark, even well south of the Arctic Circle. My guess would be that the seas are calmer then as well, but I'll defer to any expert meteorologists out there. Choice of ship will make a big difference to how much you feel the effects of any bad weather - look for reviews of any ships you are interested in, and see what they say. Once you get across to Norway, a smaller ship might stay in the shelter of the thousands of islands that stretch right down the coast, while bigger ships have to make some detours into the open sea.
If the cruise includes the west coast of Scotland (combined British Isle / Norway cruises often do) then mid May to end June is usually the best time weather wise, though being Scotland you can get snow in May and sunburned in November.
