"When evening came, his disciples went down to the lake, where they got into a boat and set off across the lake for Capernaum. By now it was dark, and Jesus had not yet joined them. A strong wind was blowing and the waters grew rough. When they had rowed three or three and a half miles, they saw Jesus approaching the boat, walking on the water; and they were terrified. But he said to them, 'It is I; don't be afraid.' Then they were willing to take him into the boat, and immediately the boat reached the shore where they were heading." (John 6:16-21 NIV)
On this passage William Still writes...
"Presumably the disciples assumed Jesus would return to Capernaum on foot, walking around the shore of the lake. Clearly the last thing they were expecting was the miracle of his walking on the water. Jesus had withdrawn to a mountain by himself, leaving the disciples guessing as to what he would do. We learn a few verses later that the crowds that had been fed with the boy's picnic lunch were also left guessing (verse 25).
"The disciples got into the boat at their Master's command (cf. Mark 6:45), not knowing what would happen to him or to them! When the storm came, they strained at the oars, making little or no progress. However, what they didn't
realize was that as the Lord prayed alone high up on the mountainside, he could see their vain struggle against the wind and waves; but not until the fourth watch (3 a.m.) did he come to them (Mark 6:48). The shock of seeing him walking on the turbulent waves when they were strenuously rowing for their lives may have demoralized them utterly. He brought to them, therefore, words of unutterable comfort.
"He still sees us in our struggles of faith in the dark, though often he delays in coming to us, while still watching over us. But when he does draw near in the stormy night, his word to our fearful hearts is, 'It is I; don't be afraid.'"
-- William Still, Through The Year With William Still, July 25 reading.
The painting above is "The Disciples See Christ Walking on the Water", by Henry Ossawa Tanner (1907).