The problem for Germany is she never had both elements of seapower at the same time; in WW1 she had the fleet but was locked in the North Sea by geography, in WW2 she had free access to the open ocean but no fleet to utilise it.
I'd also point out that the problem is very different in WW1 and WW2: in WW1 Britain had a huge coastal shipping sector taking things from one British port to another, something like 1/3 of London's needs came directly into her docks from other parts of Britain, often trans-shipped from oceananic shipping. In the 20-30s this sector dried up as railways gained more capacity, so by 1940 this coastal shipping wasn't a major factor. This created opportunities in WW1 that didn't exist in WW2, closing the channel to through traffic wouldn't have starved the nation, but it would have caused some 1/4-1/3 of London's population to be evacuated to where they could be fed.
So in WW1 if Germany had managed to change their naval geography by taking a large part of the French channel coast and ports she would have had a good change to both partially starve London and Britain. Not a war winner, but a major contributor.