Cruise ships depart from Bell Street Pier and Smith Cove Pier (which is farther away from down town). If you take light rail to down town, you will have to take a taxi to the pier unless you are traveling without luggage or are departing from Bell Street and are up for a lot of awkward schlepping.

The fare from SeaTac to Bell Street Pier is about $45 and to Smith Cove Pier it is about $65: http://www.worldtaximeter.com/seattle/seatac+airport/seattle+bell+street+pier

HTTY

Light Rail is a great new system but isn't really convenient for getting to a cruiseship. I'd just take a taxi directly from the airport. Yellow Cabs has the SeaTac contract these days, you can call them at 206-622-6500 for their price.

There is also Shuttle Express, but they charge per person, so if you are 2+ together might as well do a taxi.

To narrow things down a little, Holland America, Royal Caribbean, and Princess use Pier 91...the rest (notably Norwegian Cruise Lines) use Pier 66.

Good info from Bob, but regardless, the light rail isn't convenient to any place along the waterfront. You'd still need to get a taxi from the Westlake Station where you'd end up downtown coming in from SeaTac.

Thank you everyone for your help. I think we'll just take a taxi all the way.

Not disagreeing, suze, just pointing out that "the pier where the cruise ships sail from" doesn't help with answers.