Albert Barnes Commentary Matthew 14:26

"And when the disciples saw him walking on the sea, they were troubled, saying, It is a ghost; and they cried out for fear." — Matthew 14:26 (ASV)

They were troubled. They were afraid. The sight was remarkable. It was sufficient to awe them. In the dark night, amidst the tumultuous billows, appeared the form of a man. They thought it was a spirit—an apparition. It was a common belief among the ancients that the spirits of men after death frequently appeared to the living.