F-bird... that's a fine idea, but airboats like to explode props above about 3000. He's limited to about 3000 prop RPM.
If you're intent on sticking with a direct drive, your engine needs to peak its HP at 3000. That means a tiny cam. If you increase your HP at 3000 rpm, then you will (like F-bird says) increase prop pitch to keep it at 3000.
For the most part, direct drive applications that want more power require more cubes. The best way to increase torque (and consequentially HP) when the RPM limit is fixed is to increase displacement. That's why most direct drive airboats use big blocks. Most small block airboats with big power need to use a reducer gearbox. That way you can spin the engine to 5000 and make nice power while maintaining safe prop speed.