'Believing' cannot tip the scales in making a historical judgement about whether something really happened. I can choose to believe that George Washington threw a silver dollar across the Rappahannock, but my believing that he did it has nothing to do with whether or not he really did do it. So also with the story of Jesus walking on water: Believing that he did it has nothing to do with whether he really did do it. 'Belief' cannot be the basis for historical conclusions; it has no direct relevance.