Under propping rarely causes motors to overheat. Too much prop overloading the motor and causing it to run slower does that, as does not having working water cooling if the original was intended for it. If it isn't going as fast as intended, a smaller prop spinning faster gives a faster boat and longer run time with a cooler motor at the cost of a bit of lost accelleration.

As Hydro Junkie said, if it is a single board, or a sealed box, it is likely to be proprietary, i.e. not intended to work with standard items. Having said that, if a standard servo has been used, the black wire will likely be ground, the red wire +5 volts (or thereabouts) and the white will be signal. Or, on many, brown, red, yellow in that order.