If the cruise is a round trip Seattle cruise, getting off in Victoria could be an issue. Time was there was no problem doing this, as long as you gave the cruise line a head's up that you wanted to do so.
Lately (like the last few years), getting off in a foreign port on a US port round trip cruise (Seattle round trip, but getting off in Vancouver,, for example) has been denied. The issue is even one person getting off in the foreign port instead of back in the US port, changes the cruise from a round trip to a foreign destination cruise for everyone onboard. The customs/immigration paperwork for the latter is more involved than the paperwork for the former. Many cruise lines have opted to stop doing this and will deny early disembarkation for this reason.
If the cruise is a Vancouver round trip, and someone wants to disembark at Victoria (last port) I don't know how this is handled.