We sailed from Seward to Vancouver, B.C. from July 30 to Aug. 6 in 1999. Clear and sunny except for two days: arrived in Anchorage to find it overcast, windy and chilly. By the time the bus got to Seward it was so murky and foggy, could hardly see the nearby ships. Next morning, the sun came out, clouds burned off and Hubbard Glacier was sparkling. Great weather until Sitka where it was so foggy the tour guide had to show us a picture of beautiful Mt. Edgecomb, a volcano almost in the harbor! By early August the salmon are running and you get a good chance to see them in both Sitka's National Historic Park (fabulous collection of totem poles there) and in Ketchikan Creek at Ketchikan.
However!! the ship's captain told us to be grateful for our good weather. For the past four cruises it had been so cloudy no one could see the mountains that are so close to the water in the Inside Passage. I think the weather along the Inside Passage is pretty much a gamble. I've been in southeast Alaska in early September, and had mostly good weather, but it poured all one day in Ketchikan.
We took the Mt. Roberts tram in Juneau and hiked their one-mile nature trail at the top--can't say the mosquitoes were a problem--maybe closer to ponds and lakes they would be.
Bring "layers" as you will want all of them sailing up close to the glaciers. Mostly though, a sweater or windbreak is plenty. As a native of Juneau put it--when the sun does shine it's surprisingly warm--and humid.
The Inside Passage is ideal for cruising--the little towns have little economic base now other than the cruise ships. They appreciate the business and have made things very convenient for the ships' passengers. In Skagway, they ran the narrow gauge train right out onto the dock so you can easily climb aboard for the trip up the White Pass.
Whenever you go, I'm sure you'll enjoy it all!