For the purposes of this Act neither an internal drainage board (within the meaning of the Land Drainage Act 1930), a river board, a river authority, a river purification authority, a district board, an improvement committee, the Conservators of the River Thames nor the Lee Conservancy Catchment Board shall be taken to be a harbour authority if, apart from this provision, they would only be taken to be such by reason of the fact that river works powers or duties are vested in them; and any reference in this Act to functions of a harbour authority relating to a harbour shall be construed as not including a reference to such powers or duties.