A lot would also depend on where you are going and when. When we went up the inside passage from the end of May to mid-late June on the AMHS we had very few days without at least some rain. The inside passage is inside the Tongass National Forest which stretches from mid California up to Glacier Bay. It is also the largest temperate rain forest in the world. The weather is a lot like Oregon in the spring. We did fine with just a good, hooded rain-jacket, though we also brought pants but only used them once. Unlike just meandering through Alaska, you keep a very tight schedule on the ferry and can't wait an hour for the rain to letup when breaking camp.
Throughout the rest of the summer in the rest of Alaska we had occasional rain just like you would expect in the lower 48.