When therefore they had rowed about twenty-five or thirty stadia,{25 to 30 stadia is about 5 to 6 kilometers or about 3 to 4 miles} they saw Jesus walking on the sea, and drawing near to the boat; and they were afraid.
And they were afraid ...  The fear of the apostles sprang not merely from the weather and the dangers of the sea but also from their lack of harmony with the Lord.  It was thus intensified when they saw him approaching the vessel.