Yes, you are right, packing for an Alaska cruise is totally different than packing for a Caribbean cruise. Just be sure to include some kind of rain gear, most likely, you'll see some of that wet stuff, a simple windbreaker may be fine and or a poncho. Once there, most of the shops will be selling them. I basically wore jeans, t-shirts, sweat shirts and a jacket, also include some gloves/hats if you are planning a trip to a glacier. You basically can layer your clothing and take off a layer as the day warms up.
Here is a link to a good Alaska packing article:
Packing for Alaska