We just returned from Alaska. Based on what people told us regarding early June temperatures and rainfall, we packed rain jackets, sweaters, sweatshirts, windpants, etc. On a lark, I threw in a couple pairs of shorts and some t-shirts.
We were in port three days and all three days wore only the shorts and t-shirts. The weather was high 60's to low 70's and very sunny. It was perfect but highly unusual. On sea days, the ship was the same temperature inside as a ship in the Caribbean so the sweaters and warm clothes were useless.
So be prepared for anything! A friend went on an Alaska cruise last August and said it was cold and rained the entire time. Go figure.
definitely a jacket. we were there last sept 8-15, mid 40s - 50s the norm, except last night up in anchorage...actually snowed, about 32-35. but it will depend, of course. if glacier hopping in a helo or whale watching in a boat, downright cold, low 30s. so a jacket with sweaters, GLOVES, good idea; the secret is "layering" of clothes with sweaters etc.