Assumming the ship is docking on the weekend, the airport is only about 25 to 30 minutes away. The ship docks right downtown in Seattle and during weekdays traffic would be a consideration. I would think you could make an 11 am flight, but would book around noon just to be safe.
BTW, there is tons to see and do right around the port itself, so if you haven't seen Seattle and are not spending a day or two before the cruise, I would book myself on a flight around 6 pm, check my bags with the concierge service at the cruise terminal and go see Seattle.
There are great walking tours right from the terminal and the concierge office has maps, etc. Pikes Market is only a block or two away.