Mark 6:49

But when they saw him walking on the sea
(See Gill on Matthew 14:26);

they supposed it had been a spirit;
a phantom, a spectre, an apparition, a nocturnal demon:

and cried out;
as men affrighted at the sight, and fearing they should be hurt by it, or that it portended some evil to them; (See Gill on Matthew 14:26).