Water-whether hot or cold or kidney temperature

will only do so much in its own. It will break down some of the powder buildup so the weapon can be reloaded. (Minie balls can be a tight fit)
However at some point you need to run a brush down, to get inside the rifling to scour lead and powder residue out.
Most importantly you need to use patches and oil of some kind right after to keep the rust away.