I vaguely remember that C. S. Lewis once wrote something to the effect
that changing water into wine is something that God does regularly (e.g., by
what we call natural processes) in grapes and their fermentation, but at Cana
Jesus was showing that He is the God who does this. Whether or not it was a
more direct action I'm not sure, but presumably it was at least more obvious to
the observers Who it was making the wine.