You can, with some data and a bit of empirical testing, decide if the motor you have is adequate for the task in hand.
You need the following information.
Unloaded RPM of the motor at the voltage you wish to run at.
Pitch and diameter of the propeller you wish to use on the boat.
You can find out the RPM of any motor by using a rev counter. Optical ones are available at fairly low cost- these tend to be aimed at aeroplane use, but work just as well with boats. Once you have the unloaded RPM of the motor, you should aim for 60-75% of that figure when under load. If it's dropping to significantly under that, then you need either reduction, a smaller pitch/diameter prop, or a slower revving motor wound for more torque.
Another method is to use an ammeter. Connect this up in series with the motor and battery and measure the stall current of the motor (grip the shaft with something firm, be careful), measure the unloaded current of the motor e.g. free running. Subtract this from the stall current, the figure you are left with should be used as your reference. If you are pulling more than about 50% of the measured current bandwidth, then you need to look at the previously mentioned solutions.
The pitch of a prop can be a little more difficult to suss generally. Prop shop give excellent data with their range. Most scale props tend to be between 1.2-1.4 times the diameter of the prop.