There are two parts to the answer to your question. The first is a matter of looking at the chemistry of gunpowder and cannon balls. The carbon and sulfur will be completely unaffected by wet soil. The potassium nitrate is slightly soluble in water. It cannot be rendered more dangerous as a result.
Cannon balls are usually solid lumps of metal.
There is no way they can 'go off' unless they are in the breech of the cannon with a charge of powder behind them. It is just conceivable that a cannon might go off. A cannon ball could not. The second part of the answer is to look at the thing from the point of view of the local bomb squad. They have a very difficult, responsible, and dangerous job, and it is vitally important that their procedures are tightly laid down, and completely failsafe.