The 'rusting' happens because the water gets trapped...say, in an unlined van, or a van with wooden flooring, water seeps underneath, it's stuck there and it won't dry out... by the time it's there it is no longer pure though is it, it is now contaminated with all kinds of muck.
rust is oxidization isn't it, continual wetness + air + metal = corrossion.
This is why in potentially wet areas we have to line our vehicles properly...and not with plywood!
It isn't the pure water that is the problem, any water would be a problem.
Ian