Go by road. There are several options for taking a bus or driving to the Vancouver cruise port. You can purchase a ticket for the Quick Shuttle that runs from Sea-Tac Aiport to the Vancouver terminals by making a reservation online. You can also rent a car from Sea-Tac to drive up to Vancouver.
Take the train. Make an Amtrak reservation from from Seattle to Vancouver via Amtrak Cascades. A train travels between the cities twice a day. The train ride takes roughly three hours. From Pacific Central Station in Vancouver, you can take public transportation via SkyTrain or a taxi to the cruise terminals.
Book a flight from Sea-Tac to Vancouver. A popular choice is Alaska Airlines Horizon Air division that offers several daily flight options from Seattle to Vancouver. From the airport in Vancouver, you can take public transportation or a taxi to the cruise ship terminal.