It's interesting to recall that the plot of "The Riddle Of The Sands", by the Irish writer Erskine Childers, is based on the concept of Germany using the channels between the Baltic Islands as grouping and embarking points for wave upon wave of barges to be towed to the UK as part of an invasion programme.

"The Riddle Of The Sands" was written in 1910 so, presumably, German's intentions or possible intentions were common knowledge at that time.