Depending how long your boat is and what motor you use, you may not have to register it. This is off of the boat.wa.gov page:
Registering your Vessel
Register your boat with the Washington State Department of Licensing. To navigate, operate, employ, or moor your vessel in Washington, you must have a Washington title, registration card, and registration decals, except:
•If your vessel is a canoe, kayak, or a vessel not propelled by a motor or sail.
•If your vessel is less than 16 feet in length and has a motor of 10 horsepower or less and is used on non-federal waters only.
•If your vessel is properly registered by a resident of another state or country who uses Washington waters for 60 days or fewer.