To all. I have bad experience with water pumps. My wife had a Toyota which she bought brand new. After a couple years during scheduled service at the dealershop someone replaced coolant with Presto or some other stuff. The result... water pump blew up two years later at 50k miles. The same with my MB. Previous owner filled the cooling system with this green stuff. The result... the water pump blew up. You should see it how rusted it was.
Word of advice! Don't save money on coolant. Even if you service your car at the dealer insist on filling up with original coolant.