Unless you will be going to an excursion on a glacier, just a warm jacket, couple of sweat shirts, t-shirts will work fine. Go ahead and pack a had and gloves too, but any clothes you can layer during the day seems to work best. Oh, also some kind of rain gear, you're bound to see some of that too.
Wearing wool socks, jeans, sweater, polar fleece jacket and gloves, gore-tex parka, hat and sunglasses, I was comfortable on deck every minute of the 2+ hours we watched glaciers calving at amazingly close range. I wouldn't have missed a moment of that, not for anything! I wore just as much gear kayaking in the rain and glacier hiking on an overcast afternoon. But then again, I wore nothing heavier than a sweater the afternoon we sailed out of a sunny Seward. Have a lovely time!