Some critics say that the story of Jesus walking on the water in John 6.16-21 is out of place or the work of a later redactor. Lincoln thinks not:
“What has already been said about the distinctives of John’s account makes plain the force of this epiphany miracle for his narrative. The unlimited power of God as Creator and Savior in the Jewish Scriptures is now attributed to Jesus, who also walks on the sea as on dry land, demonstrating his control over the forces of nature, thereby reuniting himself with his followers, reassuring them and bringing them across the waters to their destination.”
“In the process Jesus is also presented as taking on his own lips the self-identification of this God in the formula ‘I am: do not be afraid.’ As in 2.1-11, where the impact of the sign on the disciples was primary, so here: Jesus’ task in this narrative is to make God known (cf. 1.18) and in traversing the sea he displays dramatically to his disciples that he is one with God in word and deed (cf. also 10.31).”
Now to the main point: is this miracle “out of place” in the Gospel of John? “In this way this tautly recounted episode can be seen not as a distraction from the rest of the chapter but as encapsulating in narrative form what is at the heart of the Fourth Gospel’s Christology” (emphasis mine).
See Andrew T. Lincoln, The Gospel According to Saint John (Peabody: Hendrickson, 2005), 220-1. (This commentary is part of Black’s New Testament Commentary series.)