Hi Lisa! The Alaskan cruise is one that I really want to do with Disney. I'm so glad you have the opportunity to take this cruise!
A week before you sail, be sure to check the weather forecast to get a more accurate picture of what the temperatures will look like. Those I know who have sailed on Disney's Alaskan cruise recommend dressing in layers. Onboard the ship, the temperatures will be climate controlled, so you'll feel comfortable no matter what inside, but a sweater over short sleeves would be a good idea.
If you wear layers, you can easily adjust to changing temperatures by peeling off a sweater to wear short sleeves on warmer sunny days.
Outside, especially in the evenings, you'll want a wind-breaker to keep you warm. I hear the breezes are very chilly as you pass through the Inside Passage to see the glaciers.
Sunglasses are also a must as it's very bright. And don't forget the sunscreen! Just because you see glaciers and snow doesn't mean you won't get burned.
Be sure to check out
Disney Cruise Mom Andi's videos (click on the "Discover Alaska" tab) with tips for your Disney Alaskan cruise. Take a look at what the guests in the videos are wearing to get a good idea for your own packing purposes too!
Have fun planning, Lisa!