Most yachtsmen will have experienced the situation of the echosounder suddenly recording a very shallow depth when passing behind a large vessel.
This is caused by the sounders signal being returned by the turbulence from the ship's wash. The first time it happens, it can be disconcerting, especially somewhere like the entrance to Lymington when the ferries frequently cause this effect.
A similar thing can happen in shallow water in rough weather. I once made a passage through the Thames Estuary in heavy weather. The channels were about 4m deep and we were surrounded by shallower water.
The water was so disturbed that the echosounder was completely useless, as the visibility was less than half a mile this made the passage quite scary!
There is not much you can do about this effect, just be aware that if you go in to a situation like this and are relying on the echo sounder to keep you off the banks it may not be much use.
Of course, the back up system of a lead line would also be difficult to operate in these conditions as well.
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