That is an interesting concept Ken.
Where did you discover that? Can you tell us the rationale for it?
I believe it's related to "Load arm length" calculations. The larger the diameter, the more force is needed to overcome it's turning resistance. Water, being heavier than air, has more frictional resistance, so the larger the diameter of the propeller the more force is needed to turn it around.
The only way to exceed these rules is to decrease the width of blade as the length increases and to increase the speed of rotation. This, of course, is undesirable in a tug as it's 'low down grunt' that will be required. More blades can be introduced as the power to weight ratio will be within the boundaries of the motor's diameter and the amount of coils within it's magnetic ring.
Or as Mikgo says, your going to need torque so a good choice of motor is essential for the job in hand.
Hope this helps
ken