We went last Sept. 8. It wasn't too cold except for the day at the glacier. Gloves & hat were in order then. We were on the Vision of the Seas, & it has an inside pool. We only had sprinkles at Skagway.
We had very mild temps last Sept, but you never know, expect anything. The pools will be cold, but there are hot-tubs that are nice and warm if you are brave enough to try those out, otherwise just use the indoor pool if your ship has one.
I found out -after- booking that the NCL Sun only has an unheated outdoor pool. I'm hoping that they heat the hot-tubs, though. On the plus side, I'm one of those crazy weirdos who can sit in a hot tub out in freezing weather. It'll be my first cruise, so I have no idea what anything's going to be like. Maybe I'll bring a wetsuit.