It bears repeating: The various electrical connections under the hood of a car are weather resistant, not water proof. Any directed water from any source risks shorts and corrosion. Hard water has minerals in it that are left behind when the water evaporates. These minerals can interfere with connections, especially high-impedance ones like O2 sensors. An inside the connectors, wires can corrode and break at the crimps making for very difficult debugging.
What value is there in "washing the engine"? I suppose if you like showing the car but geez...I didn't realize there were so many trailer queens out there. Just go around with a dry microfiber towel and wipe stuff down. Hosing or power-washing is just asking for it...
And honestly, spraying cold water on hot engine components is just asking for fractures or other thermal-shock issues. We let the engines cool completely before pulling spark plugs but dousing a hot, running engine in cold hose water is okay?