Myself, I have rarely encountered them on the inside passage and only in deep bush- wonder Lake in mid August. If you are doing heavy duty interior hiking you'll need protection, otherwise probably not.
We just got back. We had a wonderful cruise! We pretty much stayed near the water, or were inside the train, but we did not see any mosquitos. We had beautiful sunny weather but a constant cold wind. It never got out of the 40's. Have a good time!