I did the Alaska Cruise in August and needed everything - big coat, thermals, gloves, hat etc. The weather can be sunny but when you are walking on the glaciers or up close to the glaciers it is obviously very cold. I went salmon fishing in Ketchican and Juneau and was lucky with the weather I was out on the boat in a shirt. I did find it difficult to pack as I was in shorts in Vancouver and moleskins in Alaska ! Just take plenty of layers but don't forget gloves and a woolly hat. It was my first cruise and I did every excursion going which I don't regret - you must do the train at Skagway and helicoptor ride on to the glacier. If you like the outdoors Alaska is amazing - we followed a pod of Orca whales one day. I hope you enjoy it and have good weather. P.S. my mother-in-law visited the Rockies in May the year before my cruise - Banff, Jasper etc - and they still had snow at that time of yea.
We returned from our Alaska cruise 1st September, the weather was sunny for most of the time, you do need layers tho'. We went to Mendenhall Glacier and gardens in Juneau and up Mt Roberts by tram, Skagway we had a helicopter ride and walked for about 40 mins on a glacier with a guide, then after lunch went panning for gold and had Alaksan salmon( we didn't find too much gold!) Ketchigan we just went on a trolley bus which was horse drawn and a husky dog sat with us. Most tours you can book at the port of call but we pr booked ours with our cruise operator(Celebrity). It was a fantastic holiday, Hubbard Glacier the ship was about 1/3rd mile from it and we saw lots of calving (ice falling into the sea), otters, whales, eagles, dolphins etc. Enjoy I'm sure you will.