A thing to bear in mind is that when a full size boat is made, very often the prospective owner will specify the motor to go into the hull that he is buying. For any given hull, a wide range of motive power could be fitted, so there can only be a quite wide range of motors that would "suit" that hull. If making a model of a particular boat, it helps to know the actual HP of the original. This does scale very well as long as you remember that IC engine figures give power out - the figures that you find for model electric motors are invariably power IN in Watts, so due allowance needs to be made for conversion inefficiencies and whatever the shaft and prop do for, or at, you.
Long established rules of thumb that work say that the prop diameter should not exceed the motor can diameter, and the motor should have more poles than the prop has blades. This generally works for direct coupled brush motors, the coin is still up and spinning regarding brushless motors.
Back in the olde days, when there was a range of maybe 4-5 motors from the shops, they were noted as being suited for a range of hulls, and generally sort of worked. Some still do. Now that there is a vast amount of choice, there is much more scope for mistakes.