With the servo PCB modified for contious rotation, the PWM pulse you send is NOT the one bursting the motor. Rather, the PCB translates the PWM to a position, and depending on whether how far away that position is, the PCB will generate ANOTHER PWM signal to drive the motor as fast as possible to reach that position (which continuously runs away, so the motor spins continuously). This PWM signal generated by the PCB is (because of the way the servo chooses it's travel speed based on position) will certainly be nowhere near the PWM you applied to the motor. It will be very hard to find a correlation between the PWM to the PCB and PWM that is being sent to the motor.
It you need to just spin the motor in one direction (I believe that is all you need for this propeller, but you said something about reeds), all you need is a single transistor AND the % duty cycle of the PWM signal you apply directly translates to the % of battery voltage that the motor effectively sees (and there it is much easier to choose the PWM duty cycle required to drive the motor at a particular speed. No need for a servo PCB in this case, and much better control. So maybe you should weigh the much easier method with more direct control against the ability to reverse the motor to get rid of reeds.
I didn't know boats could back up...I thought the props were unidirecitonal.