Hi, it is an interesting subject however this is an area where there are multiple solutions.
I agree motor running speed should not be that low. If you know the stall current, a quite accurate prediction of most efficient running speed is 20% of this figure. So for a 20 amp stall then 4 amps will be a good load for the motor the speed needs to fit in with the load of the prop.
The problem is that there are 4 variables, motor, battery, esc, and propeller size. Most of us have settled for a particular battery size say 6 cell NiMh and the appropriate charger, this then determines the esc parameters. so the only variables left are motor and prop.
There is another aspect and that is your model will only spend 1% of it's time in the water, so is the above all that important?
Nailing your flag to the mast with just one motor could created an unnecessary problem for yourself.
There s a further school of thought that you should not run a prop whose diameter is greater than that of the motor.
My choice when doing this for a model is what prop looks right. Then find a motor to suit I am a bit spoilt for choice. In your case I would look for a pair of Robbe Navy electric motors. I am planning to put the pair I have to each drive a 4 blade 50mm prop in a Kort nozzle running on 7.2 volts All in a Mississippi Towboat 40 inches loa and about 5 kgrms disp.
If I remember right under load it was 3.5 amps at full speed, having 40 watts of power available fills my criteria for being able to stop quickly.
I have one small cargo boat that I control the speed just on the throttle trim control, for me making the right wave pattern makes the boat look right on the water.
Good Luck
Roy