I have been on two Alaska cruises. One in May and one in July It was actually cooler in July then May for me. You will definitely need a sweatshirt or something warm for the Glacier viewing. Juneau was nice and sunny and warm but the other ports could be cooler. The ship itself is usually comfortable but if you get cool with airconditioning you may want to bring jeans and a sweater or something a little warmer than shorts for when you are walking around the ship. Have a great time.