I sailed on the Diamond in Sept of last year and the weather was great!! I was glad I packed mittens and layers, thermal long sleeve tee with sweatshirt and a water proof rain slicker over everything. some days were nice and i only needed 2 layers.
For shoes i wore sneakers and waterproof clogs, and I had dress shoes for dinner. I was not on the Glaciers or any hikes, but just walking around shopping in port(s).
I'm sure you'll have a great cruise, Alaska was one of my favorite cruises. I love the hot weather and was appehensive about how cold it would be, but we did'nt have any weather extremes.
Write back and tell us how you liked it!