This just occurred to me the other day when I was thinking about components. When it's raining out and you open your door, generally you get water all over your door, and if your tweeter is mounted on your door, generally you're going to get some water on your tweeter. I was wondering if water has any severely negative effects on the tweeter.
It depends on what your tweeter is made of, silk, titanium, aluminum, etc. In either case, it's not the best situation for the tweeter. I make it a point to shield my comp's as best as possible when I must open the doors in the rain.
Most car speakers are made to be water resistant. Meaning it can handle some water splash, but not soaking. Just don't make a habit of leaving the door open during rain storm.