If anyone wants a really good product to clean their engine, check out P21S Total Auto Wash. It's pretty awesome stuff. I spray it on a completely dirty engine bay (without hosing off), let it soak, and rinse. It will be about 95% clean. Finish it off, let it dry, and apply protectant where applicable.
As for hosing, be sure to protect sensitive electronics. I like to use aluminum foil to cover things like the alternator. Just whatever you do, don't accidentally short +/- with foil... You'll have more problems than a wet or dirty engine bay then.