On general principles, any given can size for a motor will be capable of handling a given number of watts as heat to be got rid of before it cooks its surroundings and/or itself because the dissipation ability is limited by the area of the outside of the can. It is possible to increase this by adding extra area (cooling coils, fins etc).
In that size of motor there is room for just so much. This includes the magnets, the brushes and commutator and the armature assembly with the wire wound onto it. On the armature, looked at from the end, there is the metal of the armature arms and the space between, which gets filled as far as possible with wire, either a lot of turns of thin wire (high voltage, low current) or a few turns of thick wire (less volts, lots of amps). The more amps wanted (therefore power at any given voltage), the more room the brush/comm part should take up and the less space is available for the rest. All a question of balance and compromise.
To make a 15" boat go as fast as possible as long as possible with that physical size of motor, I would be looking for the smallest diameter 2 blade prop I could find to let the motor spin up while minimizing the loading. This would compromise acceleration, but the top speed that the hull could allow would be available along with a hopefully acceptable run time without melting the motor. Pitch and blade shape are two bonus cans of worms that either increase fascination or build frustration, depending.