Boats were narrow in the past mostly because they were easily driven when narrow and high power engines were not available. It's hard for me to believe most all boats are still wide (including my own) the way people complain about fuel consumption. And people don't like "tippy" boats ... even yachtsmen. Don't see any narrow inflatables. Actually I think there is one that is fairly narrow but I've never seen one.
But if your'e happy at 6 knots wide can be driven quite easily too ... almost as easily as narrow. And if my own Willy were widened to 12' beam my present engine would not need replacing and for all practical purposes Willy's not overpowered.
But if you get even close to hull speed narrow = easily driven.
North Western Washington State USA