Every waterway constructed, deepened or widened under this chapter shall, from and after the completion thereof, be a public highway for vessels and an outlet for swamp or overflow water which may be drained into it from any lands in the district or tributary thereto, and shall be under the care and control of the board of county commissioners of the county as are other highways: PROVIDED, That whenever any such waterway shall thereafter be included within the limits of any city or town, the care and control thereof shall pass to the corporate authorities of such city or town.