Jamieson, Fausset and Brown on John 6:18-19

18-19. sea arose, &c.--and they were "now in the midst of it" (Mt 14:24). Mark adds the graphic and touching particular, "He saw them toiling in rowing" (Mr 6:48), putting forth all their strength to buffet the waves and bear on against a head wind, but to little effect. He saw this from His mountain-top, and through the darkness of the night, for His heart was all with them; yet would He not go to their relief till His own time came.