This question comes from the Rhine river that is absolutely vital to German economics and even culture but the mouth of the Rhine lies in another country.
That's like the French keeping hold of Louisiana and the mouth of the Mississippi River, that would have been intolerable for the US.
The Rhine is vital to the German economy, but it was only following industrialisation that the Rhine became an "export river", rather than mainly an "import river" as it had been in the preceding centuries. When the Ruhr/Rhine area industrialised, around 1850, most of what it needed (iron, coal) was available locally or was imported from the German hinterland.
It's not really until the 1870/80s that Germany's economic potential is truly realised and the Rhine gained real vital importance for the German economy as a means to export products and import materials needed for its industry.
Use of the waterway was the same as it had been since the 16th century; toll / tariffs were to be paid in order for goods to cross.
An attempt at conquering and annexing the Netherlands (which were strategically important and politically neutral) would have brought Germany into a direct war with France and Britain; which is why Bismarck never even considered doing so.
Instead, Germany was given extensive railways and Kiel, Wilhelmshaven and Bremen were expanded.