I am playing a test game as the French side in the 4 player campaign against the AI.
When I check the river zone Rhein - that flows along Wesel - the tooltip shows that one needs 6 naval elements to blockade this zone. The Rheinflotte in Köln contains 4 naval elements.
However, when I move the Rheinflotte into the Rhein zone near Wesel, the tooltip shows "you have enough naval power to blockade this zone" (or something similar), and the harbour of Wesel shows the blockaded icon.
So, is the tooltip wrong or are there other factors that come into play?
Edit: maybe an Austrian fleet in the area? I will look at the Austrian AI faction next time.