What time does your ship board? You must be on board no later than 1 hour prior to sail away and there are no exceptions as you must be present for muster.
The normal drive from SeaTac to downtown is 45minutes if traffic is good. However I'd check to see if there is anything going on downtown before feeling that's an okay drive time. There are football games, baseball games (all downtown), festival, Sea Fair, etc.
Please tell me your times for boarding, day and date you are leaving and I'll try to be more specific. BTW: Taking a cab from the airport is more reliable than the bus.
Awaiting your response,