Pity that common sense can't provail.....
As mentioned, there's far more potential for upset by wake when off the plane.
If it's a Bye-law, the direction to mariners doesn't seem to contain many specifics of what constitutes a breach of the law.
For example, on Loch Lomond, there is a rule regarding slow speed within 150m of the shore, one for speed within defined areas, one for not travelling above 30kph when towing, one for not travelling above 90kph flat out. All very objective, and easily understood.
The direction to mariners from Clydeport appears to encompass all the elements of what would be deemed to be good seamanship anyway.
As always, it's the majority who suffer because of the stupidity of the minority, like the nutters on PWC chasing the Arran Ferry last year, one of whom I watched fly off the wake, dumping the two in the water at which point their machine landed about 2 feet away!!
It's only two weeks ago, I watched Clydeport's own boat 'Cruiser' going through the area in question, producing a tug boat sized wake!
Anyway, here's a question...At what point does a yacht under power or sail produce no wake?!!