We are also on this sailing as well! Hope you find this info I dug up helpful!
SEATTLE: I live in Seattle (grew up in Anchorage) and we are in the throes of an El Nino summer which, for us, means dry and hotter than usual (it's going to be in the 90s all week). I am wondering if that will affect the weather in Alaska since I can't find any data around El Nino in AK. However, for your embarkation, late August/early September is still a warm time in general for Seattle with 70s - 80s not uncommon.
JUNEAU: In general, you should expect high 50s-low 60s for the high in Juneau. It can get down into the mid 40s at night. Rainfall can be from 6-7" per month at this time of year.
SKAGWAY: Skagway should be upper 50s-mid 60s with mid-upper 40s at night. They get from 2-4" of rain per month at this time of the year.
KETCHIKAN: Ketchikan should be 60-65 in late Aug/early September with an average of 50 at night. Rainfall averages 10-13" this time of year so prepare for wet.
VICTORIA: Victoria should be mid-upper 60s with little rain (.5-1").
What you should pack sort of depends where you are from and how acclimated you are to mild-cool weather. These temperatures are pretty close to an average late spring for us in Seattle so I am basing my packing around that. Native Alaskans will be in shorts and t-shirts. But, if you are from a warmer climate, temperatures might seem positively frigid to you requiring warmer gear. Everyone should definitely prepare for rain.
Also, tailor your clothing to the excursions you are planning. Obviously, walking around town isn't going to require anything special, but if you are going out on a boat for fishing or whale watching, or hiking on one of the glaciers it will get chilly. Also, if you are going anywhere forested (hiking, fishing, biking) I have one word for you - mosquitoes. Wear long pants, long sleeves, socks and bring the repellent.
Cabs from SeaTac to downtown Seattle are $30-35 (before tip). I haven't checked to see which terminal the ship leaves from, however. If it's from the downtown pier, it should cost about that to get to the pier. If it's the terminal a little further north, it will be a bit more expensive, but not too much.