We also did an Alaska trip, although we did it in June. We also did a walk on the glacier from Juneau - we were provided with coats, boots & crampons before we set off in the helicopter, yes it was cold, but when we did the walk it was bright sunny skies and the guides had t shirts and shorts on - no coats ! Like I said, it was June though. Juneau itself was hot and we were in t shirts - I have no idea what August weather would be like. On board when scenic cruising past the glaciers, it can be cold and damp, and we did need our hats etc, the mist tends to come down at times and it can be quite cold so we were glad of them. Ketchikan is known as the rainy capital - there's a mountain permanently shrouded in a mist cloud and it was a little wet but not cold. I'd say the best thing is to take layers, you can off load or wrap up when needed, but a lightweight rain mac would be a necessity I'd say. The scenery is fantastic - you'll have a great time - enjoy your cruise !