In managing and controlling a harbor, the board of supervisors has authority, upon such terms and conditions as the board shall prescribe, to grant franchises, permits and leases for the use of lands and facilities belonging to the county, and to prescribe the rate or rates of wharfage and other charges which are appropriate for the use of any of the facilities owned or services furnished or provided by the county. All revenues derived from such franchises, permits, leases, and charges, are available solely for payment of the cost of maintaining harbors that are under the control and management of the board of supervisors, except that the board may from time to time, out of any surplus of such revenues, transfer to the appropriate funds of the county such sums as in its judgment may be available for payment of principal and interest of county bonds issued pursuant to the provisions of this article.