One other thing that I forgot to mention is dressing in Alaska. The best way to go is to dress in layers. Temperatures can change quite a bit as the day wears on. Generally a sweater and light jacket are sufficient. Also be sure and bring rain gear as there probably be at least one day when you will need it.
The post about needing winter coats when near a glacier is not correct. Temperatures in the upper 30's, in summer, would be anything but typical. If you were standing right on the Glacier this claim might be correct, but on a cruise you will be at least a quarter of a mile away from the glacier. You will find that the temperatures might be 10 degrees cooler when you are near a glacier but that is about it. Unless you are really affected by cooler temperatures, IMHO, a winter jacket, hat and gloves would be way over kill. Maybe someone who has been to Alaska can offer an opinion about this. I was on the Island Princess which began on May 11 of this year and the weather was great, with temperatures a little cooler than our past cruises, but still I found a sweatshirt and light jacket were plenty.
Another thing to consider is that if you take a cruise that goes to Hubbard Glacier in May or June, is that the ice flows sometimes keep ships from getting anyway near the Glacier. In College Fjord and Glacier Bay getting close to the glaciers is not a problem, even the first cruises in early May. As I said before, for me, Glacier viewing is the highlight of any Alaskan cruise.
Princess, Holland America and a couple of other cruise lines do offer many cruises with two days of glacier cruising with one in College Fjord and then another day in Glacier Bay National Park. If seeing glaciers is a priority, you might want to consider a cruise line that offers two days of glacier cruising.
Debra