I doubt it was the water.
1. It was running fine.
2. You "tore it down to the frame. (Cleaned it and put it back together again.)
3. Now it won't start at all.
But... 4. You "touched the battery terminals together, and nothing sparked."
Do you mean that you shorted directly across the terminals of the battery itself, and got no spark? If so, then yes, your battery is toast.
If that is not what you meant, then you have to go back and look again at every connection, everywhere. Because if the battery has juice and you are getting nothing anywhere else, then either a battery cable is internally broken, or you have a bad connection somewhere.
Will the tractor start if jumped from a running car?