Your boat needs high current over a short period of time. Look at a capacitor for short term peak power. It may take several seconds to charge, but it can discharge very quickly.
Consider a low voltage motor, perhaps a motor rated for 1.5 - 3 volts. The ability to give up heat is the general limiting factor for motor ratings. With your extremely short duty cycle you can push the motor almost to the point where the current melts the windings.
I'd consider a prop with enough pitch that the motor rpm is loaded down. DC motors typically provide full torque at zero rpm. A slower turning prop may be more efficient than a high speed prop.
Power to weight ratio is critical to good performance. So keeping the boat light will improve acceleration rate.