Engine power vs rpm curves are for the maximum output of the engine. At anything but wide open throttle the power to move the boat is significantly less that the maximum engine output for the rpms you are running at. The figure below shows some typical power curves for a hypothetical boat for a typical diesel engine. The power to move the boat is shown in red (solid and dashed) and the engine maximum output curve is shown in black. Your premise was basically, correct, but you were using the wrong curve. The actual power curve for each boat must be measured or calculated from parameters such as prop details, gear ratio, displacement, wetted surface area, etc. The calculations are simple if you have the hull line