We have done two Alaskan cruises. One started in Seattle and returned to Seattle for a 7 day cruise.This is the only one I'd consider doing.
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Second one was a flight to Anchorage, stay for a couple of days to visit the area and take the train to the Cruise ship. End up in Seattle.
The second trip was far superior because you get to spend the entire day in each port on the one way cruise vs just a few hours on the round trip cruises.
Always fly into Anchorage on the right side of the plane because you get to see 5 glaciers on final approach and book the left side of the cruise ship when going south so you're always looking inland vs looking at open sea on the right side of the ship while heading south.