Any such wharf, pier, dock, bulkhead, building, harbor or other structure, constructed, altered, improved or extended within the limits of any municipality beyond low water mark, without a license or order of court first obtained therefor, shall be deemed and taken to be a public or common nuisance and the person causing the same to be so constructed, altered, improved or extended without a license, shall be guilty of maintaining a nuisance, and upon conviction, shall be sentenced to pay a fine not exceeding five hundred dollars, or to imprisonment of not more than six months, or either or both, in the discretion of the court, and the court may, by its order, direct either the defendant or the sheriff of the county, at the expense of the defendant, to abate the nuisance. Every such license or permit or order granted or made for the construction, alteration, improvement or extension of any such wharf, pier or other like structure, beyond low water mark of the waterways, or any harbor structure, shall become and be absolutely void, unless work thereunder shall be commenced within six months after the date thereof and diligently prosecuted to completion.