Wash. Rev. Code § 36.08.010
If a harbor, inlet, bay, or mouth of river is embraced within two adjoining counties, and an incorporated city is located upon the shore of such harbor, bay, inlet, or mouth of river and it is desired to embrace within the limits of one county, the full extent of the shore line of the harbor, port, or bay, and the waters thereof, together with a strip of the adjacent and contiguous upland territory not exceeding three miles in width, to be measured back from highwater mark, and six miles in length, and not being at a greater distance in any part of said strip from the courthouse in the county seat of the county to which the territory is proposed to be annexed, as such county seat and courthouse are now situated, than ten miles, a majority of the qualified electors living in such territory may petition to have the territory stricken from the county of which it shall then be a part, and added to and made a part of the county contiguous thereto.
The petition shall describe with certainty the bounds and area of the territory, with the reasons for making the change and shall be presented to the board of county commissioners of the county in which the territory is located, which shall proceed to ascertain if the petition contains the requisite number of petitioners, who must be bona fide residents of the territory sought to be stricken off and transferred to the contiguous county.
If satisfied that the petition is signed by a majority of the bona fide electors of the territory, and that there will remain in the county from which it is taken more than four thousand inhabitants, the board shall make an order that a special election be held within the limits of the territory described in the petition, on a date to be named in the order.
Notices of the election shall contain a description of the territory proposed to be transferred and the names of the counties from and to which the transfer is intended to be made, and shall be posted and published as required for general elections.
RCW 36.08.010