Wash. Rev. Code § 35.21.160
The powers and jurisdiction of all incorporated cities and towns of the state having their boundaries or any part thereof adjacent to or fronting on any bay or bays, lake or lakes, sound or sounds, river or rivers, or other navigable waters are hereby extended into and over such waters and over any tidelands intervening between any such boundary and any such waters to the middle of such bays, sounds, lakes, rivers, or other waters in every manner and for every purpose that such powers and jurisdiction could be exercised if the waters were within the city or town limits. In calculating the area of any town for the purpose of determining compliance with the limitation on the area of a town prescribed by RCW 35.21.010, the area over which jurisdiction is conferred by this section shall not be included.
RCW 35.21.160