There is a great dispute amongst critical writers what this fisher's coat was; whether a loose coat, or the garment next his skin, or a fisherman's slop. It is a point not worth the disputing: it was some garment that might modestly cover him when he came to Jesus, and yet not hinder him in his swimming.
Related Commentaries of John 21:7
John 21:7
7 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.