I went on a cruise to Alaska last July on Princess and everyone dressed very, very casually. For day, it was jeans, long sleeve T's and a sweater with a lightweight, removable lined jacket (one good for rain, so a hooded one works best). Layers are best, but I never took any of mine off! Of course, socks and good, comfortable walking shoes. Bring a hat and gloves for when you do an excursion to a glazier (highly recommend the helicopter to a glacier to do a dog run!) At night, black dress slacks and multiple tops and one dress for formal nights. No one swam or hung outside at night (too cold). We had beautiful weather the whole time we were there and the temps were about 55-65 during the day. It was so funny to see the men walking around town in Talkeetna without shirts on because they were hot! It was 65 that day. We also did a land excursion to Mt. McKinley and Denali; both locations were jeans appropriate, even at dinner at the lodges. The most casual, laid back vacation we ever took and our favorite so far!