A couple important points:
1. Never, ever, never never NEVER
run water in your radiator. Proper coolants have stuff in there to prevent corrosion. ALWAYS use a good antifreeze/coolant. Corrosion is most likely the cause of many of these blown headgaskets.
2. A blown headgasket will not make the motor blow white smoke. A blown headgasket will put steam in the exhaust. There is a big difference, and it matters. White smoke in the exhaust is caused by oil, and does not disperse. It comes out the tailpipe and is carried away by the wind or hangs in the area. If you start a motor making white smoke in the garage, it will fill the garage with white smoke.
A motor with coolant in the exhaust makes steam in the exhaust. It's completely different. It disappears quickly after it hits the air . Steam in the exhaust is only there until it cools. Steam in the exhaust will not build up, and you can't watch it be carried away on the wind.
Yes, steam in the exhaust is white, but it looks nothing like white smoke whatsoever.
3. Coolant in the exhaust has a distinct smell. Even a teeny tiny bit of coolant will be quite noticeable.
Think you have coolant in your exhaust? Go stand next to your tailpipe and take a whif. If there's coolant in that exhaust it will be obvious.
What does coolant smell like? Good question and I'm glad you are paying attention.
Take your bottle of antifreeze/coolant (You aren't putting water in your radiator, are you? surely you have some antifreeze, right?) and pour a little bit into the cap.
Now take that cap full of antifreeze and pour it on your hot exhaust manifold. There. That's what antifreeze smells like.
Now that you are familiar with the smell of antifreeze you can use that to help locate where the coolant is going.
Good luck and have fun!