Is there
any part of the bolt sticking out, with the water pump removed?
If so, you're in luck. You can take a pair of vice grips, tighten them down onto the part that's sticking out, and hopefulyl be able to thread the broken part out.
If there isn't anything sticking out, you can take a drill and "drill the bolt out", if there's physically space for the drill. Just make sure that the bit you use is a few sizes smaller than the bolt itself...and if the surrounding parts don't collapse and come right out, you may need to drill it to the size of the bolt and re-tap the hole.
If you don't have a tap-and-die set, I don't think they're much more than $30...and it definitely beats having the car hauled away, especially if that's all that's wrong with it.
