A.T. Robertson 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 (εταραχθησαν). Much stronger than that. They were literally "terrified" as they saw Jesus walking on the sea.

An apparition (φαντασμα), or "ghost," or "spectre" from φανταζω and that from φαινω. They cried out "from fear" (απο του φοβου) as any one would have done. "A little touch of sailor superstition" (Bruce).