Alaska cruise vacationers experience temperatures that range between 50 and 70 degrees most of the time. You should be comfortable in long pants and a sweater, so bring several of each. Also, pack a windbreaker to avoid getting a chill. Bring a couple of short sleeve shirts just in case. Juneau, in particular, can reach into the 70's and feel downright balmy.
Pack a raincoat. Alaska is wet in the summer. Even when it's not raining, it is misty and you will get damp. Don't let icky weather keep you inside.
Speaking of damp, bring at least two pair of outdoor shoes in case one pair gets wet. If you plan on doing some serious hiking, bring a good pair of hiking boots. If you plan to just stroll around, bring a couple of pairs of good walking shoes.
If your cruise itinerary includes a view of a glacier, it will be very cold, especially in the early morning. So, bring one winter jacket, a pair of gloves, and a winter hat. You don't want to be stuck viewing the glacier through the glass.
Check the dress code for the cruise. Alaska cruises usually schedule one or two formal nights, so bring at least one formal outfit and several semi-formal ones. Also, check to see if they have a heated pool. You may want to bring your bathing suit.
