For the purposes of this Act neither an internal F4drainage board, the Environment Agency, the Natural Resources Body for Wales, a waterF1F2undertaker, F3the Scottish Environment Protection Agency a district board nor an improvement committee 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.