Why are you removing the water pump drive pulley ? Did the tensioner pulley freeze and cause the belt to burn up the cam pulley ? The water pump pulley comes with the new pump - you don't need to remove it from the old pump assembly.
If your taking the pulley off the cam shaft extension DO NOT try to tap or drive it off or back on with a hammer or any other tool other than a water pump puller, you will be suprised how easily that extension will snap off the end of the cam if you bang on it. The pulley needs to be drawn off and pressed back on. Ask me how I know. Yes I broke an extension off trying to tap a pully back onto the shaft. You will think that the camshaft is made out of platinum when you see how much GM wants for a new one, if they are still available.