Sandy is quite right. Layering is definitely the way to go in Alaska! The weather can vary greatly; often rainy, but not always. On a 10-night May cruise along the Inside Passage, we experienced everything from 45 to 65 degrees, from steady rain to occasional drizzle to glorious sunshine. We were very glad we had brought rain gear - both slickers and also special rain pants - and used them as additional windbreakers as well as in the rain, as we spent most of our time outside on deck to enjoy the scenery. We had gloves, winter hats, multiple sweatshirts, plus windbreakers and slickers on - all at the same time - and managed to keep from freezing.
The only downside is, when you want to go back inside for a spell, you have to peel off at least half the layers or you will roast!!