The first place to start is the other end of the drivetrain: the propeller. The Resolve and Sir Lancelot are scale boats so the correctly-scaled propeller size should be on the plans. Then you look at what voltage you would like your propulsion system to run at, based on what you want to use for batteries and speed controllers. The last thing you do is look at the maximum effective RPM of your chosen propeller (provided by Raboesch, or
here and pick a motor that spins close to that RPM at your chosen voltage. The hard thing to determine is how much torque per amp you need; nobody has published a graph of how many gram-centimeters or inch-ounces it takes per mm of prop diameter. Generally, if it's a slower-turning motor, it's wound for torque over RPM, which is what you want.
This way, you have a model that has a wide range of operational speeds without worrying about not enough power at max throttle or cavitation at higher power settings, and which doesn't draw excessive amps just cruising around the pond at half-power.
EDIT: should mention that the best way to figure out torque is to eyeball the motor's diameter and see that it is close to the diameter of the propeller
