I never cover anything but the air filter, as mine is semi-exposed. Water has never caused me any issues in almost 4 years of doing it this way for either the alternator or the coils/ICM. Do you know how much road spray is in your engine bay in a heavy rainstorm in the winter?
Use anything but a pressure washer. Common engine degreasers work great, follow the directions on the can. Use a small brush for detail work. It might take you 2 or 3 tries the first time you do it.
You may want to use a detailing spray afterwards. CD2 leaves a protective film that bakes on, making subsequent cleanings much easier.