If you look at the bottom of a Zodiac, it's not just flat. It's got a hint of a keel on it for maneuverability. Not a proper hull though.
Hull design is an advanced science. In the days of sail it was as important to national security as aerodynamics are today. A hull was a necessary evil: all that ship down under the water gave you lots of drag, but without it the rest of the ship wouldn't float.
Then we invented outboard motors and all that science was made irrelevant by raw power. You could turn a bathtub into a high performance speedboat by bolting a big enough motor on it. When the throttle is high, the impact of the water against the bottom of the hull lifts it right up out of the water. It skims like a skipping rock and who gives a fuck about hydrodynamics. When you throttle it down, the vessel sinks into the water again and wallows like a hog.
Neal Stephenson