A.T. Robertson Commentary John 21:7

"That disciple therefore whom Jesus loved saith unto Peter, It is the Lord. So when Simon Peter heard that it was the Lord, he girt his coat about him (for he was naked), and cast himself into the sea." — John 21:7 (ASV)

It is the Lord (ο κυριος εστιν). John's quick insight appears again.

Girt his coat about him (τον επενδυτην διεζωσατο). First aorist middle (indirect) indicative with which note διεζωσεν εαυτον in 13:4. Apparently Peter threw on the upper garment or linen blouse (επενδυτην) worn by fishers over his waistcloth and tucked it under his girdle.