Water pump bolt broke off

Grind the bolt flat on the end. Be careful not to hit gasket surface. Next use a center punch and make a divot dead center. Use a small drill bit and drill through. Increase the bit size and drill again. Once you get it drilled you can try an EZ out. If you have a small torch you can carefully heat what is left of the bolt cherry red and let it cool before using ez out. The heat will expand the bolt and break the corrosion. You can also spray some penetrating oil around the threads as its cooling. The oil will wick into the threads.
That's an easy one, should be out in 10 minutes or less.