Alaska information

As a starter, I would suggest checking on the internet for the September average high/low temperatures for your port of calls. You can also find average percipetation info (amount of rain) on yahoo weather. I would guess you would go through Inside Passage and south eastern Alaska, it could get really wet there. Would suggest: 1. layering what you wear so you can easily add and take off as needed.
2. Have something weather/water resistent - like a Gortex coat, hopefully with a hat.
3. Anytime you get near the ocean, you almost always need a jacekt/coat or some kind of wind-shield. And that's true even in summer in Seattle and Vancouver.
4. To give you an idea, the average high and average low temperatures in Juneau, their state capital, is between 44 and 56 in September with a 7.5 inch rainfall, on average the 2nd rainiest month of each year.
5. It also depends on what activities you pick up on the trip: are you going hiking, kayaking, landing on a glacier in a helicopter?
Have a nice trip.