When we did this cruise two years ago the weather was largely dry, sunny, but on the cold side (around 8-10c during the day) most of the time. Certainty take a fleece, warm coat and thick socks for going ashore (layers) - it will be warm enough on board the Infinity - which is a beautiful ship by the way. The scenary is amazing outside Skagway for example and the town is really interesting if you like the old wild west. A float plane trip over the glaciers is a must or one of the wild life or train trips. You should have a great time.