A district may improve navigable and nonnavigable waters of the United States and the state of Washington within the district; create and improve for harbor purposes new waterways within the district; and regulate and control all such waters and all natural or artificial waterways within the district and remove obstructions therefrom, and straighten, widen, deepen, and otherwise improve any water, watercourses, bays, lakes or streams, whether navigable or otherwise, flowing through or located within the district.