Most of the cruises in Seattle depart from a common terminal in Seattle, that is, on waterfront from Pier 66.
And most cruises use the Seattle cruise ship terminal as a jumping off point for the different cruises going to Alaska.
The waterfront of Seattle is found to be generally very crowded because of not just the cruise ships but also because of the Seattle ferry system.
http://www.destination360.com/north-america/us/washington/cruise-terminal-seattle-waterfront
The two terminals at the Seattle cruise terminal are the Old Bell Street Pier Cruise terminal and the newer terminal that has been created is Terminal 30. The Celebrity and the Norweigan cruise Lines are served by the Bell Street Terminal and Holland American and Princess Cruise Lines are served by Terminal 30. A Seattle to Alaska cruise is there from all these major cruise lines. This also includes the Silver sear Cruises.
The Glacier Bay cruise line leads in being one of the most popular Seattle to Alaska cruise.
This cruise line has two spring cruises moving to Alaska. There are also two trips to Seattle while returning from the Seattle to Alaska cruise.