Originally Posted by rockyp
There is no ferry from Seattle to Vancouver. You could take the Victoria Clipper ferry to Victoria on Vancouver Island and then another ferry to Vancouver, but that is highly impractical. Driving from Seattle to Vancouver (bus or one-way car rental) is the only other option, which takes 3 to 4 hours, IIRC.
The train is another great option. I'm not sure about getting from the Vancouver station to the airport, but it shouldn't be too hard. Train fare is $28. There is one train and 4 amtrak buses you can take.