I was on HA Alaskan (Sept.) cruise 4 yrs ago, and on RCL to Alaska last year in June. Yes, I found that jeans were fine for shore excursions. Otherwise, I wore slacks & sweaters or nice shirt during the day. Weather in Alaska can vary greatly from port to port in June. I tend to be cold-natured, but most days (in ports) I wore a lightweight turtleneck, shirt, sweater or sweatshirt over that, then a windbreaker-type jacket that is water-resisitant. (The jacket came from Land's End catalog.) I also wore silk "longjohn" top at least one day.
I am sailing to the Western Caribbean this June - but envy you! Alaska is my favorite destination on earth! Please forgive my P.S.', but I thought I'd pass along:
* make sure everyone traveling with you packs binoculars - A must for bear sightings in Glacier Bay
* my personal favorite excursions - Juneau / floatplane trip to Taku Lodge for a salmon bake. Worth every penny! www.takuglacierlodge.com
& in Skagway / the White Pass Railroad trip
* Whale watching out of Juneau or Sitka - unbelievable!!
Have a ball!