The "idler pulley" is used to apply constant tension to the serpentine belts used to drive the alternator, water pump, a/c compressor, etc. on most cars today. as long as it didn't disintegrate the water pump shouldn't be a problem. Waterpumps usually wear out on their own.
Just replacing the tensioner (idler assembly) shouldn't run more than about $200
Idler pulley is what takes the slack out of your belt, the pulley itself should be about 20-30$ if you have to buy the whole tensioner it'll probably be 50-75$