The easiest way is to use Quick Shuttle.
They go between Sea-Tac and Vancouver in about 5 hrs.
The earliest shuttle leaves at 6:30AM and arrives at Canada place
at 11:10 and ballantyne pier at 11:20.
the link is
I would highly suggest that you arrive a day early and visit Vancouver.
Check to see if the cruiseline has transfers from Seattle to Vancouver(rtn).