Jesus walks on the water and intends to “pass by” his disciples. What’s going on here? Well, he’s almost certainly modeling his behavior on an Old Testament pattern where references to controlling the waters are combined with God “passing by” the people (see Exodus 33:19-34:8, Job 9:8-11, and 1 Kings 19:11). In other words, he does what God does: he controls the waters and he passes by his people as a way of revealing himself to them. So at the beginning of this story, the audience has every reason to believe that the disciples are about to learn that Jesus is doing what the God of the Bible does.
But that isn’t what ends up happening. The disciples see Jesus but they do not realize that it is him–they think that they are seeing a ghost. (The KJV translation of “spirit” doesn’t really work here–this isn’t the word that Mark uses for spirits, either holy or unclean. This word means “ghost.”) I am pretty sure that anyone listening to the Gospel of Mark for the first time in the ancient world would have laughed out loud at this, and not just because the disciples scream in terror. The reason this line is so dang funny is that we have a lot of ghost stories from the ancient world and guess what is the one thing that their ghosts couldn’t do? Walk on water. So the response of the disciples is completely absurd. It’s like finding someone in your kitchen chopping garlic and deciding that he must be a vampire. It’s like performing surgery to remove a silver bullet and deciding that your patient must be a werewolf. It’s the one conclusion that just doesn’t make any sense given the facts available to you. You know one thing only about the cook and the patient, but that one thing is the only thing that could make the conclusion preposterous. And that is the conclusion that the disciples reach about the “ghost.”
Well, Jesus’ plan to reveal himself as God obviously can’t happen in the face of these terrified, screeching, completely obtuse disciples. Instead, Jesus says, “Have courage! It’s just me! Don’t be afraid!” Except that isn’t precisely what he says. The “it’s just me” part uses precisely the same words as Exodus 3:14. In that story, Moses asks God what he should say that God’s name is should the people ask. God responds: “Tell ’em ‘I am.'” Those words for “I am” (in the Greek translation of Exodus) are exactly the words that Jesus uses. So . . . Jesus ends up revealing his identity to the disciples anyway. It’s, you know, ironic. The disciples don’t get it, but the audience does. (That’s how most of Mark works: the disciples are slow learners, but the repeated lessons give the audience ample opportunity to learn.) The revelation of God in Jesus happens, despite the disciples.
The miracle in this story has precious little to do with Jesus’ ability to walk on water. It has everything to do with Jesus’ identity. He plans to reveal his identity to the disciples, but their slow-wittedness derails his plan. Except that it doesn’t. The real miracle in this story is Jesus’ ability to overcome human obstacles to teach what he intends to teach, which is that he is the Son of God. Jesus appears to abandon his plans in order to help his ignorant and screaming disciples, but because he is Jesus, he is still able to accomplish those plans, for the benefit of the audience if not the disciples.