I went both times the 3rd week in August & hadreally good weather both times. Carried a parka with me when I left the ship but hardly even used it...usually wore a sweater with a Tshirt or blouse under it so I could take the sweater off. On board ship, I just wore short sleeve pants suits -I'm not a shorts wearer. On the NCL ship I did wear sweater & sometimes parka the day we were in with the glaciers.
I'm a TA & if you're going this year, book it TOMORROW! Not much left...many cruises are totally sold out. Alaska is really hot this year.
One thing to consider - my 2nd trip & also my parents 2nd trip was on the Dawn Princess. Great ship & we had a terrific time but we were very disappointed because the ship was too big to get in close with the glaciers. When I went on NCL & when they went on RCCL, both ships were small enough (still hundreds & hundreds of pax + crew) to be much closer to the glaciers. It was cold outside - that made for great atmosphere (you could run right inside to thaw out)...sounded like a war with all the noise from the glaciers. It was great.
Last 2 weeks of May & first 2 weeks of Sept. (then the season's over...) are the least expensive times to go.
Try to allow for an extra day to enjoy Vancouver - great, great city.