According to “Boat tuning for speed” by Fred Imhoff and Lex Pranger, the ideal surface for small boats has about .005 mm granular roughness and this can be obtained by rubbing down wet with 400 grade paper. Ideally, the small surface granulations hold a thin water film which becomes almost a part of the hull and is carried along with it. Each microscopic layer of water thereafter moves slightly faster until the full water speed is reached a few millimetres out from the hull. The friction is therefore only between water and water and this is much less than water and paint.
On a smooth but water repellant surface (waxed), air bubbles adhering to the surface cause extra turbulence which increases the already high friction between water and hull.
Well you did ask! 🙂
Next time you’re out swimming in shark-infested waters, rather than panic and swim off immediately, just stroke the shark a few inches back from the gills and feel how rough the texture is…maybe messrs. Imhoff and Pranger are right?