Whenever the county legislative authority of any county with a population of one hundred twenty-five thousand or more deems it for the interest of the county to engage in or to aid the United States of America, the state of Washington, or any adjoining county or any city of this state, or any of them, in construction, enlargement, improvement, modification, repair or operation of any harbor, canal, waterway, river channel, slip, dock, wharf, or other public improvement, or any of the same, for the purposes of commerce, navigation, sanitation and drainage, or any thereof, or to acquire or operate wharf sites, dock sites, or other properties, rights or interests, or any thereof, necessary or proper to be acquired or operated for public enjoyment of any such public improvement, and to incur indebtedness to meet the cost thereof and expenses connected therewith, and issue bonds of the county for the payment of such indebtedness, or any thereof, such county is hereby authorized and empowered, by and through its county legislative authority, to engage in or aid in any such public work or works, operation or acquisition, as aforesaid, and to incur indebtedness for such purpose or purposes to an amount, which, together with the then existing indebtedness of such county, shall not exceed two and one-half percent of the value of the taxable property in said county, as the term "value of the taxable property" is defined in RCW
39.36.015, and to issue the negotiable bonds of the county for all or any of such indebtedness and for the payment thereof, in the manner and form and as provided in chapter
39.46 RCW, and other laws of this state which shall then be in force, and to make part or all of such payment in bonds or in moneys derived from sale or sales thereof, or partly in such bonds and partly in such money: PROVIDED, That the county legislative authority shall have first submitted the question of incurring such indebtedness to the voters of the county at a general or special election, and three-fifths of the voters voting upon the question shall have voted in favor of incurring the same.