315.11 Exemption from taxation.—As adequate port facilities are essential for the welfare of the inhabitants and the industrial and commercial development of the area within or served by the unit, and as the exercise of the powers conferred by this law to effect such purposes constitutes the performance of proper public and governmental functions, and as such port facilities constitute public property and are used for public purposes, the unit shall not be required to pay any state, county, municipal or other taxes or assessments thereon, whether located within or without the territorial boundaries of the unit, or upon the income therefrom, and any bonds issued under the provisions of this law, their transfer and the income therefrom (including any profit made on the sale thereof) shall at all times be free from taxation within the state. The exemption granted by this section shall not be applicable to any tax imposed by chapter 220 on interest, income, or profits on debt obligations owned by corporations.