I used to take my old air-cooled VW Bug to the car wash, cover the carb and distributor with plastic bags, and let loose a mf-ing blast of hot soapy water to get all the grime off the motor and tranny. Cleaned it like a charm, and it ran cooler as a result. Only drenched the distributor once and had to blow it dry to start.
Not sure I'll ever get that brave with the Prius, but if the air intake and HT leads are kept dry (or dried before starting), I don't think there would be a problem. You might get a DTC or two if the water changes a resistance reading of some sensor, though.
Hopefully it won't leak oil like the Bug and so won't need it.