score:6
The cruise ship terminal in Vancouver is a couple of kilometers away from Pacific Central station; you could walk in about half an hour, or take a taxi. I'm not sure why your search failed, but King St. Station in Seattle is connected to the International District/Chinatown station on the light rail system, from which you can catch a direct ride to SEA airport; the trip takes about half an hour, plus waiting time.
However! Cross-border service on the Amtrak Cascades is currently paused. Connecting bus service may be available, but it does not look like there is any currently offered at the times you suggest. Amtrak's website says service will resume "in early 2022", so it is possible you may get lucky and service will be available at the time of your proposed trip, but it is not certain at this point.
Note also that, when cross-border train service is available, US customs and immigration processing takes place in Vancouver, so you are advised to get to the train station an hour before your departure time. I do not know how strict they are about this.
Upvote:4
Is what I'm proposing manageable in a single day (I don't think it is, we probably need to overnight in Seattle)
Yes. Ample time for it.
then there are flights at 3:30pm or 5:30pm
The last non-stop SEA->SAN flight departs at 8:55 PM.
and if so how do we get from King Street Station to Sea-Tac?
King Street Station --1-min walk--> Int'l Dist/Chinatown Light Rail Station --light rail--> SEA.
Upvote:6
As another option: the "Quick Shuttle" bus will take you directly from the Canada Place Pier to Sea-Tac Airport (with several intermediate stops but no transfers). According to their schedule, they leave Canada Place at 9am and arrive Sea-Tac at 2:35pm. A search in late April says they're offering tickets for $23.50; I'm not sure what is typical. (I am not affiliated with this bus company; I have heard of them as a good option for cross-border trips between BC and Washington state.)