Walking on water was something the disciples had never seen, and they were very terrified.  Their understanding said that it was physically impossible for a human body to walk on water, so they concluded it was a ghost.  Their terror was voiced by their screams.  It is bad enough to be in a storm, without seeing a ghost in the middle of it.  In our storms, fear may grip us too.  In our nights of terror, in the middle of some storm, we need to hear His reassuring words.

But when they saw him walking on the lake, they thought he was a ghost. They cried out, because they all saw him and were terrified. Immediately he spoke to them and said, “Take courage!  It is I.  Don’t be afraid” Mark 6:49-50 (NIV).