I used to work for a company that blew out slips using propwash....not all that easy to do without strong currents in the area...and just pushing it to a neighbors slip that would now have to be done which is worse from your slip. Its dangerous as the forces exerted to the pilings or docks was enough to snap lines...even old pilings and you had to be ready to react in a second. If it did happen...if you weren't using a well fendered workboat to dredge...you wish you were at that point.
Pretty much all the outfits I know doing it stopped maybe 10 years ago when the EPA went after a USCG crew for doing it down in North Carolina
when they couldn't get the ready boat in and out of it's slip at low tide.
Yep ....permits and money
are the answer...mainly because those choosing not to do it will generally rat you out for destroying the environment
or pushing sand/silt where they don't want it.