I've been to Alaska the first week of Sept. and it was rainy and chilly one day, then warmer the next. I would take mostly long pants and tops, waterproof windbreaker and a couple of light layers. Maybe one set of shorts/t-shirts. But, I am a total wus about cold weather. It was definitely warmer than I expected. It also depends where you are going, which ports. Alaska is a BIG state. And the weather is different. Ex. Sitka never goes below freezing, even in winter.
Wish I could go too!