Check your belt tensioner. I have a '98 e300. The alternator pulley on my car came apart, and I lost most of it when my car was just idling in heavy traffic. When I put things back together, I discovered that the tensioner pulley was bad, apparently a failed bearing. I think that problem caused the problem with the alternator pulley. Presumably, it could also cause problems with a water pump.

I'm just a DIY guy rather than a real mechanic. I just want to give you my experience so you can check to see if you have the same problem I had.