The board of supervisors may enter into any contract for the use, sale or disposal of such waters that in their judgment shall be for the best interests of the district; but no such sale, contract or disposition shall be made except by the unanimous vote of the board. The district shall not guarantee nor warrant the amount or flow of, nor the title to, such waters; and no use, sale or disposition of such waters shall be lawful that will interfere with the efficiency of said drainage system.