For the purposes of this Act neither an internal drainage board (within the meaning of the F1M1Land Drainage Act 1976)F2F3the National Rivers Authority, a water undertaker, a river purification authority 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.