Just got back from Alaska. I agree with much of the above. The Spirit does have the roof closed unless the weather is really warm. You can for sure use the pools/hot tubs. Deck 3 is a great covered deck to be outside, but out of the rain if you happen to have a rainy cruise week.
Everyone says layers. Every cruise will vary, but when in the fjords, you will likely need an underlayer, a t-shirt, and a long sleeve (flannel, sweatshirt, etc.) However, this is going to depend 100% on the weather you are used to. It was around 50 degrees in Glacier Bay last week. We were in t-shirts (midwest residents) and some were in winter coats (coming from Florida.)
If you have a great naturalist, their lectures are worth attending! They will give great insight on what to look for / watch for to help you spot that whale spout or the cute sea otters! If you are interested in nature and history, they also will have a lot of great information to share with you!
Binoculars are a great thing to have along! Scanning the shore line and ocean for wildlife is a great way to enjoy your day!
Have a great time!