[SRGN]

Remove the belt and see if any of the accessories, water pump, or idler/tensioner pulleys are making noise or binding on their own. If anything driven by the belt has too much resistance it will prevent everything driven by the belt from being turned at the proper speed including the water pump. Do what Chris said, check the water pump for radial and in/out play, and see if it binds up at all when you apply force to it from the side and try to turn it.