I think part of the question can be answered by where you live. We are from Chicago and were fine with a sweatshirt covered by a light jacket. If you normally live in Florida or Arizona, I would then recommend something heavier. It also depends on what type of excursions you are going on. If in doubt, I would go lighter, but bring a hat and gloves.
It also depends if this is just an inside passage or all the way to Anchorage with a land tour. If the latter, then you may want to include something warmer. Just remember to also bring warm weather clothing. We had temps in the 80s in Ketchikan on our first July cruise in Alaska.