There shall be three commissioners to the New England Interstate Water Pollution Control Commission, appointed as herein provided. The Commissioner of Public Health and the Commissioner of Energy and Environmental Protection or their representatives designated in writing shall also be members. On or before July first, annually, the Governor shall appoint a commissioner who shall hold office for a term of three years from such July first. If any vacancy occurs in the office of any appointive commissioner, the Governor shall fill such vacancy for the unexpired portion of the term. The commissioners shall serve without compensation, but shall be paid their actual expenses incurred in and incident to the performance of their duties.