Sunprincess is right. I can't say it enough: layers - layers - layers. You could get 80s, or you could get 50s (or anywhere in between). You could get sun, or you could get rain. With nonstop scenery, you will want to be outdoors, up on deck or on your balcony (whichever applies)a good bit of the time. I'd suggest a windbreaker in addition to a rain slicker or poncho (the latter preferably with a hood); you would wear these over sweatshirt/sweater, and t-shirt or longsleeved shirt. You can always un-layer if the weather gets warm. I'd suggest a hat, possibly earmuffs if your hat and/or hair doesn't cover ears too. And sunglasses - even on overcast days, it can sometimes be rather bright.
If you are on the standard cruise ships and not the tiny 100-passenger ones, there will be a couple formal nights, so you will need some dress-up clothes (like you'd wear to a wedding, or to a swanky theater) for those nights.
If it's your first-ever cruise, there is a cruise packing list on a sister site, www.cruisereviews.com
- go to Cruise Q&A in the lefthand column on that site, and scroll down to the link for the packing list. Very helpful!
Have a fantastic time! "Beautiful" doesn't even begin to describe Alaska!