The greatest ports of Germany are located on this little North Sea coastline or on its rivers--Hamburg and Bremen.

It might therefore be assumed that any nation which successfully blockaded these North Sea ports would have strangled the commerce of Germany.

That is far from being the case.

The point is that the political boundaries of Germany are simply fictions, when economic considerations are involved.

Holland, on the west, and Denmark, on the north, are as much a part of the German transportation system as though these two countries were parts of the German Empire.
Their territories and the territories of Germany are contiguous; the railroad and the canal systems of Germany, Holland, and Denmark are practically one.