The key to dressing for Alaska is to remember that the weather in Alaska is like a box of chocolates -- you never know what you're going to get.
That's why everyone will advise you to wear layers.
I went in early July 2009 -- the weather was "seasonable".
Here's a link to my trip report.
Alaska Review w/ pics and tips! 7/11 - 18/2009
Enjoy your cruise!!!
You are going to have a great time!