According to the quick search, the sign means that the boats must slow down not to generate the backrush that would make the boat hit against the quay.
thanks, that sounds reasonable.
In channels the suction pulls the boat at the pier away, then it hits the quay wall again, pulled back by the ropes and backflowing water
We have speed limit in those places, sometimes for long distances even in rivers, as well for saving the embankment
