That is everything everwhere always.
If you look at the aerdynamic signature of your average speedboat, you'll find the drag coefficient is comperable, if not less, then you average drop top car. Moving through the air, that thing was a miata. Now, you have the hydrodynamic drag of the water, which is a significant force, yes. But when a boat like that is up to plane, it probably has about 6-10 square feet of contact with the water at most. The amount of drag on a boat at 16 is much less then a boat at 20. So wht is it that hatchabck festivas can hit 100 mph with 100 hp, but a boat like in the vid needs 500? Because boats are screwed, litteraly. You use hp much more efficiently when you're driving wheel. When you're putting it down through a pair of screws, inefficient isn't even the word. Put a diff and some wheels under that boat and it'll do way more then 100mph on the highway...if it dowsn't lift off the ground.
You're speed on the water is reliant on your screw design and how fast you can spin it. If you have a singly drive, 3 screw tranny, you need a bunch of torque just o spin the gears and overcome the parasitic loss, hens to offerings that Joe mentioned. For something small like what is in the vid, the motor should have no trouble spinning a single, or even a double screw setup sufficiently.
In reality we shouldn't even be attempting to discuss this untill we know what power the boat in the vid actualy makes, anf if its spinning a prop or jet drive.
Edit:
http://www.xtremerotaries.com/main2/Quicksilver.htm