Bring money and plastic-see everything and buy everything-you will not regret it!
Seriously, be prepared for cool mornings, cool-to-mild days and cool nights-dress in layers. We encountered temps from the high 40s to low 70s but it can get warmer. We used combo of sweatshirt or sweater with windbreaker. Coldest time was on a high glacier outside of Juneau on in Glacier Bay with breeze and drizzle. Gloves and ear muffs would have been nice in both places. We went in 97 in August and only had one day of rain in Glacier Bay-but chance of rain is high anytime, specially from Ketchikan northward. On 7 day HAL cruises there will be 2 formal nights, probably 3 casual nights and 2 informal nights. Most people dress up for formal nights-tuxes/business suits for men and long/evening gowns for ladies. The ship will have full dry clean/press service (for a fee unless you are in a suite). Bring comfortable walking shoes and be prepared for wet/damp ground conditions both on ship and on land. Umbrella or rain gear is advisable. I took a back pack for daytrips and excursions to hold windbreaker, hat, trinkets, unbrella, camera stuff, etc. Take lots of film. Take a bottle of water with you and just refill it on ship to take with you for excursions. Binoculars are a plus for this cruise-lots of wildlife.
Since sailing to Alaska, we have taken 2 more HAL cruises and all our tablemates who have been to Alaska all agreed it was the best cruise they had taken.
We are planning to go back next summer on HAL with a bunch of friends. Have a great time!