Try a Hobbyking Turnigy 2836-3700kv (700watt) runs on 3-4S batteries on a 35A ESC.
I am running one on an outboard with a 32mm x 1.4 pitch prop, only on 3S(2650mah)(41070rpm).
The boat is about the same size around 600mm(24 inches).
It's insanely fast and with your boat setup with water cooling the motor mount the motor would be very reliable.
Timing I set all mine on the highest with start power 10%
I get a nice controlled start off speed and to cruise back to the landing jetty..
Reverse 25%.