this is from the maritime hand book
No Wash Signs
“Wash” is the wave effect created by a vessel moving through the water.
No Wash signs are placed in some areas where the wash from a vessel is
likely to cause damage to the foreshore or vessels, or injury or annoyance
to people. Travel at a speed which creates minimal wash when you see
this sign and when near moored or anchored vessels. Look behind you
to check that you are not creating wash and note the effect of your wash
on other boats and the shore. Adjust your speed if necessary. Regardless
of signs, you should not navigate your vessel in such
a way as to produce excessive wash that endangers
other vessels or causes nuisance, as this is an offence