A little ambiguous to say that any small motor will do the job, as they come in so many different winds from mild to wild.
Instead it's much better to think about what diameter and pitch of prop you wish to run and go from there.
In the case of a scale sub, the style (e.g. blade area and shape) and diameter of prop tends to be dictated to us by the original 1:1 prototype, and if we wish to remain faithful to the prototype, then realistically the only thing we can adjust to suit our requirements is the angle of attack of the blades of the prop to influence pitch.
Generally the optimum range is between .8-1.5 times the diameter, with 1-1.25 being about optimum.
If we know the pitch of our prop, we can quite accurately calculate the speed our boat will make at a certain RPM. We also need to bear in mind that electric motors work at best efficiency when loaded at about 75% of their unloaded RPM, so we should ensure our motors do not operate at below 75% to get best value. If we go above that figure, then the motor is getting an easy life, but that is preferable to it being bogged down, where much of the energy is transferred into producing heat.
So how do we know what motor to select? Well the way I go about it is to consider the speed I wish the boat to travel at, then I calculate the pitch of the prop if it isn't already published, then work out how many RPM that prop needs to get the required speed. I then see what motors are available to me that can give me that RPM plus 25%. If running direct drive the diameter of the motor should be within 3/4 the prop diameter or greater a rough guide but effective. Very often it's not possible to get a motor at sufficiently low RPM, therefore you need to think about some form of reduction (gears, pulley etc.).
All this will get you very close to the magic number, but there is still a fudge factor, as hull shapes, propeller efficiency etc. all influence the final performance, but it's so much better than guess work.