Been to Alaska twice on HAL The last time was just two years ago in May. We has beautiful weather. So you could experiance either extremes. In Juneau I actually didn't even need my jacket it was so warm. Our trip was on the Zaandam and the roof was opened over the pool and people were actually sunbathing. Of course around the glaciers it was cooler and my winter jacket worked well with gloves. Just bring layers and you can add or take off as needed. As for formal nights, only saw a few tuxes. Most men just wore suits. It is a more relaxing cruise as the highlights are really the scenery.