As a preliminary to the agreement mentioned in section 13:8-23 of this title, the board shall require that the waters of the fresh water lake or pond mentioned in said section 13:8-23 be actually free to all citizens for a period of at least six months and notices thereof conspicuously posted on the shores of such lake or pond. The board shall also require a covenant that the lake or pond will not be closed to the public without notice given to the board and by signs posted at least six months in advance of such intended closing. When the board shall be satisfied that the waters of such lake or pond are actually free for the use of the public, subject to the provision made in said section 13:8-23, it shall certify to the assessor of the township or other municipality in which such lake or pond is taxable, that, by virtue of the agreement made between the owner and the board, the property has become an adjunct state forest reserve, and by the provisions of this section and said section 13:8-23, is exempt from taxation.