Therefore that disciple whom Jesus loved saith unto Peter, It is the Lord. Now when Simon Peter heard that it was the Lord, he girt his fisher's coat unto him, (for he was naked,) and did cast himself into the sea. His fisher's coat - Or, his upper coat. Επενδυτην, from επι, upon, and ενδυω, I clothe; something analagous to what we term a great coat or surtout.
He was naked - He was only in his vest. Γυμνος, naked, is often used to signify the absence of this upper garment only. In 1 Samuel 19:24, when Saul had put off his ἱματια, upper garments, he is said to have been γυμνος, naked; and David, when girded only with a linen ephod, is said to have been uncovered, in 2 Samuel 6:14, 2 Samuel 6:20. To which may be added what we read in the Sept. Job 22:6, Thou hast taken away the covering of the naked; αμφιασιν γυμνων, the plaid or blanket in which they wrapped themselves, and besides which they had none other. In this sense it is that Virgil says, Geor. i.:299: Nudus ara, sere nudus, i.e. strip off your upper garments, and work till you sweat. See more examples in Bp. Pearce.
Cast himself into the sea - It is likely that they were in very shallow water; and, as they were only two hundred cubits from the land, (about one hundred and thirty-two English yards), it is possible that Peter only stepped into the water that he might assist them to draw the boat to land, which was now heavily laden. It is not likely that he went into the water in order to swim ashore; had he intended this, it is not to be supposed that he would have put his great coat on, which must have been an essential hinderance to him in getting to shore.