I can only say that our boat on a mooring, with 3, 4" cowl vents moving air through, stays fresh below. I see boats on moorings with inadequate ventilation leaving hatch boards out. I doubt that works. Air doesn't move that well without large channels promoting a flow. I think it's the positive air flow, actual movement, that keeps a boat dry below. You can have mildew right next to an opening that isn't moving air through.

Our setup wouldn't work on a dock, the cowls wouldn't be nearly as effective.