The answer is that you might get the odd day when T shirts are ok but be prepared for cooler weather and probably some rain. Ketchikan for instance has 23 feet of rain a year and a lot of the others are not far off this. It will be very cold if you go up The Pass at Skagway. Take layers of clothing that way you can always take stuff off (and some rainwear). Just to brighten it we had a superb cruise, the last one of the season, in September with some brilliantly warm days and some wet days and some cold days so don't despair it's a great place to go. ...Nei.
We cruised in September on the same ship and wore t shirts, but on some days needed a sweater. Cruising up to Hubbard Glacier needed every amount of warm clothing you had got!!!! Did go in the outdoor pool too. Very mixed temperatures, but overall very pleasant. I would suggest you take clothing as if you were holidaying in the UK!!! (a bit of everything.
It can be quite warm so t-shirts may be the order of the day. Take plenty of insect repellent as the mosquitoes(non-malarial) can be ferocious. It's a beautiful place.