''This Act shall, save as otherwise appears from its subject or context, extend (a) to the ports mentioned in the First Schedule and to all parts of the navigable rivers and channels leading to such ports respectively to which the Indian Ports Act. 1908. extended on the 1st April 1937.1 and (b) to the other ports or parts of navigable rivers or channels to which the President of the Union, in exercise of the power hereinafter conferred, extends this Act. But nothing in section 31 or section 32 shall apply to any port, river or channel to which the section has not been specially extended by the President of the Union...''