Section 18A. Upon petition of ten citizens of the commonwealth that in their opinion public necessity requires a right of way for public access to any great pond within the commonwealth, the department and the attorney general or a representative designated by him sitting jointly shall hold a public hearing and receive such evidence thereon as may be presented to them. The joint board may make such additional investigation as it deems desirable and if it appears to said board that such a right of way exists it shall present a petition to the land court for registration of the easement. If it appears that no right of way exists it shall submit a report, together with recommendations thereon, to the general court on or before January first of the following year. This section shall not apply to any body of water used as a source of water supply by the commonwealth or by any town or district, or water company, nor shall it affect the right of the commonwealth or any town or district or water company to the use and control of the waters of any such pond for the purposes of a water supply, nor shall it affect or diminish any existing right to the use of the water of any such pond for mercantile or manufacturing purposes.