Tis...
I know the law because I lived it for 2 years...
I owned a Donzi 22 Classic with a Blackhawk drive on it... in gear at idle the boat went 7-8 MPH GPS. Guess what... no ticket from the MP once I demonstrated that was as slow as the boat would idle in gear thus maintaining steerage...
Perhaps you should consult a lawyer on how the law is actually interpreted. I did when I owned the Donzi 22...
Woodsy