And when evening came, the boat was out on the sea, and he was alone on the land. And he saw that they were making headway painfully, for the wind was against them. And about the fourth watch of the night he came to them, walking on the sea. He meant to pass by them, but when they saw him walking on the sea they thought it was a ghost, and cried out, for they all saw him and were terrified. But immediately he spoke to them and said, “Take heart; it is I. Do not be afraid.” And he got into the boat with them, and the wind ceased. And they were utterly astounded, for they did not understand about the loaves, but their hearts were hardened. When they had crossed over, they came to land at Gennesaret and moored to the shore. (Mark 6:47–53)This is not simply a great party trick to show off his power. This is, in the words of John’s Gospel, a sign—a powerful deed with a meaning. And that meaning was very much tied up with the meaning of the sea. The Jewish Scriptures had spoken of God, the powerful creator, as the one who walks on the sea, exercising his great power over it: “[God] who alone stretched out the heavens/ and trampled the waves of the sea” (Job 9:8). The Greek translation of Job 9:8 says that God “walks about on the sea as on the ground.” Mark uses this same word for Jesus walking about (peripateō) on the sea. Jesus is acting out the role of God in Job 9!
Jesus, like Jehovah, exercises complete control over the chaotic sea and walks upon its waves as on the dry ground. And in case ancient readers were dumb enough to miss the point, Jesus’ words of comfort to his disciples in verse 50 should clarify things. Jesus says, literally, “Be confident, I am.”This is deliberately ambiguous. On the one hand, it simply means “Take heart, it is me, Jesus.” On the other, it is an allusion to God’s self-designation, “I am.” In other words, in this story Jesus is acting as Jehovah himself, Lord of the Sea.— The Biblical Cosmos, pages 43–44
So, when did Jesus become God in the eyes of the disciples? A lot earlier than many scholars think! That's part of the reason why the crucifixion was such a shock to their theology.