Jamieson, Fausset and Brown on John 21:7-11

7-11. that disciple whom Jesus loved, said, It is the Lord--again having the advantage of his brother in quickness of recognition (see on Joh 20:8), to be followed by an alacrity in Peter all his own.

he was naked--his vest only on, worn next the body.

cast himself into the sea--the shallow part, not more than a hundred yards from the water's edge (Joh 21:8), not meaning therefore to swim, but to get sooner to Jesus than in the full boat which they could hardly draw to shore.