Personally, if you have an arrival time at 11, I would just go to the port. If you try to drop off bags early, you are fighting traffic for disembarking passengers, and porters will be tied up with them. There also could be 2 ships in port, which makes it worse. Add to that a weekday embarkation, and you also have rush hour traffic.
There really isn’t anything close to Pier 91 for people watching, so you would have to go back to downtown Seattle, and then back to Pier 91.