Section 17. No license or other authority to build structures upon or to fill up or enclose any ground mentioned in the two preceding sections shall be construed to interfere with or impair the right of any person affected thereby to equal proportional privileges of approaching low water mark or one hundred rods from high water mark, or harbor lines established by law, or to impair the right to obtain a license or authority so to approach of persons having interests in lands or flats which may be affected thereby, or to impair the legal rights of any person. All things done under such license or authority shall be subject to the approval of the department. If the general court establishes a harbor line within the outer line covered by such license or authority, the same shall be limited by and not extend beyond such harbor line. This section, so far as may be, shall apply to licenses granted under section fifteen to erect structures on great ponds.