Propeller efficiency (both and air and water) are factors of torque, RPM, and advance speed (effectively the speed of the vehicle). You can't be efficient for all conditions. For example, a tug boat needs really high thrust at low and even zero advance speeds as it pushes up against a giant ship like a tanker. That design would be very different from what you'd need for a high speed boat with high advance speed. Similarly, props for drones can be designed for efficiency at hover (zero advance speed), or for going fast. Designing for noise will also have trade-offs for different conditions. Like it might be really quiet AND efficient in hover, but then be crap at when moving forward at high speed. Efficiency is always about trade-offs.
And one of those tradeoffs can be complexity. For example, you could integrate a variable pitch propeller with a control computer and your throttle to ensure you are always at the highest efficiency possible within the geometry constraints of the propeller.
