It depends where you are starting from as Vancouver was about the same temperature as London, but you will need very warm clothes for the cruise. We went in May this year and you definately need a hat and gloves and a very warm coat.
If you want to know what to do in each port please go to this site that I direct every one to - Try google and 'cruise ports call boards' - click on the the first result will give you all the info under the appropriate ports section on the page under Alaska
You will need layers .
I bought an expensive duvet jacket but don't think I actually wore it on the cruise. I had a TOG 24 aquataine lightweight wind and waterproof jacket and a pair of light overtrousers, both from an outlet and very reasonably priced, and was truly comfortable at all times. A zip in fleece or a couple of lightweight fleeces with other thin layers did the job. We had good dry but cool days and wet windy and cold days but were constantly told, on tours, of the 160" rain that Alaska has each year. Hat, gloves and a scarf are essential, especially when cruising towards a glacier, but one set of outerclothes is ample.
A very informative comment and well worthy of following.