Why cover anything? I mean, especially the alternator. It gets soaked when you drive around in the rain anyways, it isn't that sensitive to getting wet. I hose down the engine every time I wash it, and if crap is particularly caked-on then I use some of that orange-based cleaner. After many muddy days and hours on air-filter-choking dusty roads my engine compartment still looks brand new. Go ahead and hose it -- just make sure you do it when it's cold, not hot!! Hot engine + water = bad!
