The Pulpit Commentary on Matthew 14:27

But straightway Jesus spake unto them ( ἐλάλησεν, not ἔκραξεν). He was evidently near them. Saying, Be of good cheer ( θαρσεῖτε, Matthew 9:2); it is I be not afraid. Encouragement, self-manifestation, recall from present terror.

But the absence of θαρσεῖτε in John 6:20 suggests that it is, perhaps, a duplicate rendering of the Aramaic for μὴ φοβεῖσθε. For the LXX. commonly translates "fear ye not" by θαρσεῖτε (e.g. Exodus 14:13; Exodus 20:20).

One or two second-rate manuscripts omit θαρσεῖτε in Mark, but this may be only due to a reminiscence of John. It is also omitted in Tatian's 'Diatessaron' (edit. Hemphill).