“(2) For the purposes of the area defined in subclause (1), a straight line joining the mean high-water marks at each side of a harbour entrance or, as the case may be, the natural entrance points of a river mouth must be treated as the inner boundary of that area if it extends seawards or as the outer boundary of that area if it extends landwards, except in the case of any harbours and rivers where specific co-ordinates are provided.”