The guy I know who has a commercial herring license for dip netting works out of an open aluminum skiff. Since I can't be on the boat with him and haven't hung around long enough to watch him at work I will opine that he waits and watches until a herring ball is pushed to the surface and then moves in to dip. I suspect stealth in approach is important to not scare off whatever is pushing the herring to the surface.

As to cast nets......still illegal inside - probably as you suggest due to efficiency against the daily limit.