When they had rowed three of 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 in the boat, and immediately the boat reached the shore where they were heading.
To the disciples being caught on the lake at night in a storm was truly terrifying. They were not amateurs; several of them were fishermen and knew exactly what it meant. When they saw what they took to be a ghost, they must have feared for their lives.
Jesus' attitude to the storm was quite different. He knew that he had absolute control and treated the walking on water as nonchalantly as he would walking on the street. He had disdain for all the weather could throw at him.
Matthew has an interchange between Peter and Jesus in which Peter tries to walk on water, but sinks because of his little faith. AS if there had not been enough evidence at the feeding of the 5000 the walking on water incident forces them into the declaration that Jesus is the son of God.
The more I think of it, the more this incident demonstrates the theme of John's gospel: The old has gone: the new has come.