Oh wow, that is unfortunate indeed. Hopefully you did just pop a "freeze" plug and did not crack the block. I think that might be the case if the water is leaking out that fast. But, keep in mind that normally the "freeze" plugs don't guard against freeze damage. This is true because the "freeze" plugs are actually casting hole plugs that have nothing to do with guarding against freezing.
Now, go heat that garage so you can fix the problem and prevent similar damage in the future. I wont mention anything about NEVER using water alone in the coolant system as you have learned your lesson about one of the two reasons. The other being poor lubrication of the water pump bushing/bearing. Oh crud, did I mention it...
