A.T. Robertson Commentary Mark 6:51

"And he went up unto them into the boat; and the wind ceased: and they were sore amazed in themselves;" — Mark 6:51 (ASV)

They were sore amazed in themselves (λιαν εν εαυτοις εξισταντο). Only in Mark. Imperfect tense picturing vividly the excited disciples. Mark does not give the incident of Peter's walking on the water and beginning to sink. Perhaps Peter was not fond of telling that story.