A.T. Robertson Commentary Mark 6:50

"for they all saw him, and were troubled. But he straightway spake with them, and saith unto them, Be of good cheer: it is I; be not afraid." — Mark 6:50 (ASV)

It is I (εγο ειμ). These were the astounding words of cheer. They did not recognize Jesus in the darkness. They had never seen him or any one walk on the water. His voice reassured them.