When a vessel enters a port subject to this Act, but does not discharges or take in any cargo or passengers therein (with the exception of such unshipment and reshipment as may be necessary for purposes of repair), she shall be charged with port-due at a rate to be determined by the [Government] [Substituted by the A.O, 1937 for "Local Government "] and not exceeding half the rate with which she would otherwise be chargeable.