Grounding to the "hull" means only 2 precise locations. The 2 ground points to the hull are the negative battery grounds which in most cases are right between the engines just below the firewall. All grounds for everything should ultimately ground only to those 2 points.
The idea is to eliminate a network of grounds throughout the hull.
You can have ground buses all around the boat but they all must isolated from the hull and run directly to the negative battery ground points and only at those points should they be grounded to the hull.
Its convenient to ground instruments etc to the hull wherever they are being installed but that is a bad idea.
The only thing I've seen grounded right to the hull is the trim tab motor. I'm assuming the factory did that and its not something that runs for long periods of time, they probably figured it wont hurt anything. Probably a little corner cutting there.
-Karl
1986, 32' Sedan, twin 360ci, 275hp Chrysler's w/ K&N flame arrestors