The travel document requirements are the same for all US citizens, whether your home state is Washington or not (where many ships sail from Seattle). Although US and Canadian passport holders, when crossing the border by sea or land, are not currently required to sail with a passport on most domestic itineraries (including Alaska), we HIGHLY recommend that they are carried.
If your cruise requires international air travel, such as a flight between the US and Canada, you will be required to present a valid passport when departing from or returning to the US by air. Even if your cruise does not involve air travel outside the US, should you be required to unexpectedly depart a vessel prior to the end of the sailing, a passport would be required to re-enter the US by air.
But technically, on domestic itineraries for which a passport is not required, US and Canadian passport holders may present an enhanced drivers' licence as valid documentation (note that this means you must own a passport, even if you don't bring it).
Since neither your cruise agent nor the cruiseline are responsible for verifying these type of travel documents, the best place to search for further information is Customs and Border Protection -- here's a link: http://www.getyouhome.gov
Bottom line, to cover all your bases and for peace of mind, a passport is the way to go!