From: John 6:16-21
Jesus Walks on the Water
 When evening came, His (Jesus') disciples went down to the sea,  got into the boat, and started across the sea to Capernaum. It was now dark, and Jesus had not yet come to them.  The sea rose because a strong wind was blowing.  When they had rowed about three or four miles, they saw Jesus walking on the sea and drawing near to the boat. They were frightened,  but He said to them, "It is I; do not be afraid."  Then they were glad to take Him into the boat, and immediately the boat was at the land to which they were going.
16-21. It seems the disciples were disconcerted because darkness had fallen, the sea was getting rough and Jesus had still not appeared. But our Lord does not abandon them; when they had been rowing for some five kilometers (three miles), He arrives unexpectedly, walking on the water--to strengthen their faith, which was still weak.
In meditating on this episode Christian tradition has seen the boat as symbolizing the Church, which will have to cope with many difficulties and which our Lord has promised to help all through the centuries (cf. Matthew 28:20); the Church, therefore, will always remain firm. St. Thomas Aquinas comments: "The wind symbolizes the temptations and persecution the Church will suffer due to lack of love. For, as St. Augustine says, when love grows cold, the sea become rougher and the boat begins to founder. Yet the wind, the storm, the waves and the darkness will fail to put it off course and wreck it" ("Commentary on St. John, in loc.").
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.