Couple of dumb questions:

Did you use the little rubber washer?

Did you torque down the manifold before you tightened the water pump to manifold bolt? You need to tighten the manifold bolts and tighten the water pump bolt sort of at the same time. Also, you can use a little silicone on the rubber washer.

If you did all these things and it still leaks, something is out of whack.