Flying to Seattle & Sailing out of Vancouver
Not a problem as long as your flight gets into Seattle before 11am and your ship sails in the evening. However my recommendation always is to arrive in the port of departure the day before just in case of airline - luggage delays.
To get from SEA airport to Vancouver there is a bus shuttle service called Quick Shuttle that operates 5 times daily. Cost is $44. CA pp each way. The trip takes about 3.5 hours and the bus drops you right at the dock. The Amtrak train leaves SEA at something like 6am for the trip
northbound to Vancouver . Another option is a one way car rental - again driving time about 3.5 hours including time for the customs (barring any huge lineups).
Your return flight would need to be late in the day if the same day you get off the cruise ship.
I would suggest no earlier than 6pm.