This is just my thoughts on this matter...
To give a boat a good turn of speed.. alone this statement could mean anything..
Getting a boat like the dog boat to plane would need a certain amount of rpm..
Most normal propshafts are only recommended upto 10000 rpm..
You might have to make your own propshafts with ball bearing 's both on the motor and prop side.. to take the rpm that you require..
Or one plays with the size of prop..
Of course a brushless motor works on rpm according to the amount of volts applied.. wether a out runner or a in runner
An out runner your cables would have to be neat and tidy, kept away from the motor..
An in runner gets warmer than an out runner, but you could use water Cooling...
I would go for a brushless that has torque...