Albert Barnes 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)

Therefore that disciple whom Jesus loved. John 13:23.

It is the Lord. He was convinced, perhaps, by the apparent miracle, and by looking more attentively at the person of one who had been the means of such unexpected and remarkable success.

His fisher's coat. His upper or outer garment or tunic, as distinct from the inner garment or tunic which was worn next to the skin. In Peter's case, it may have been made of coarse materials such as fishermen commonly wore, or such as Peter usually wore when he was engaged in this employment. Such garments are common for men in this occupation. This outer garment he probably had laid aside.

He was naked. He was undressed, with nothing on but the undergarment or tunic. The word does not require us to suppose a greater degree of nakedness than this. See Mark 14:51 and 1 Samuel 19:24.

Did cast himself into the sea. With characteristic ardor, desirous of meeting his Lord again and showing his affection for him.