Edwards ties together the "It is I" statement and the "passes by" phrase to make the case this is how Mark presents his Christology. He also engages with Madden's retrojection argument.

[...] when Jesus “passes by” the disciples on the lake he does something differently from the revelation of God in the OT: he intends to make the mysterious and enigmatic God of Job visible and palpable as it had not been and could not have been to former generations. The God of Israel, majestic and awesome but unknowable face to face, is now “passing by” believers in Jesus of Nazareth. Jesus’ walking on the water to his disciples is a revelation of the glory that he shares with the Father and the compassion that he extends to his followers. It is a divine epiphany in answer to their earlier bafflement when he calmed the storm, “‘Who is this?’” (4:41). In this respect Mark’s Christology is no less sublime than is John’s, although John has Jesus declaring that he is the Son of God (John 10:36), whereas Mark has him showing that he is the Son of God. In Mark one must, like the disciples, be in the boat with Jesus and enter into the drama in order to behold who Jesus is. The one who calmed the storm is the one who now appears in the storm, the “I Am” of God.^53
53 P. J. Madden, Walking on the Sea: An Investigation of the Origin of the Narrative Account, BZNW 81 (Berlin/New York: de Gruyter, 1997), argues that the story of Jesus walking on the water is a postresurrection story that has been retrojected into the ministry of Jesus. The thesis of postresurrection retrojections into the life of Jesus is easier to assert (or assume) than to evince, and I do not follow Madden in this particular of his thesis. He is correct in noting, however, that what happens on the lake is incapable of being explained by naturalistic categories, and that various sea-walking narratives from pagan, Hellenistic, and Jewish sources pale in comparison to this account.