Hm, I'm sure everyone else who has posted here is correct.
However, I've always taken my car to the serf-serve washes and sprayed degreaser/water right onto everything under the hood, without covering anything up. I've never had a problem. I try not spray directly onto the alternator of course, but I'm sure plenty of fluid gets on there. I don't keep the car running while I do this either...
I also keep the pressure of the sprayer low.
I wouldn't recommend it if people have actually experienced problems after doing something like this. But it's always been fine for me.