BAD idea.. Cold water on a hot engine block WILL break it.
The best way to clean under the hood is to start with a cool engine, cover your electronics and filter, spray a good quality de-greaser / engine cleaner on, (gunk makes a good one, buy several cans and use liberally.) Let it soak into the crap, heavy deposits might require a little brushing / scraping or at the very least another shot of degreaser once you rinse the engine.
Let the degreaser / cleaner soak in and penetrate the crap. Then hose off.. Try NOT to soak any of the electronic components and MAKE SURE your alternator is covered during this.
Degreaser / cleaner will clean the oil out of bearings. Its not uncommon for your idler pulley to go bad if you degrease it. (thats cause you washed the grease out with the cleaner.) The same thing WILL happen to your alternator and thats one expensive part to replace.
Other than that, its just common sense to keep the degreaser / cleaner away from things like the air filter or open intake.
Engine running? NO. Off and cool. (Doesn't have to be ice cold, but should be cool enough to rest your hand on without being burned.)
Who told you that you could hose down a running engine with a hot block? Some F'ed up ricer who wants you to destroy your car ? Or just some prick that wants your business when things go bad?
Dave T. - 1987 Cavalier RS/SS Wagon - 384,000 Mi - Long Island, NY
"Bob Hope, Johnny Cash, and John Ritter.. Rest well and thank you."