"It is the Lord!"

7 That disciple whom Jesus loved therefore said to Peter, “It is the Lord!” When Simon Peter heard that it was the Lord, he put on his outer garment, for he was stripped for work, and threw himself into the sea (Jn 21:7).

Jn 21 is one of the major accounts of the post-Resurrection appearances of Christ. Liberals typically discount the historicity of John because it's too theological. Mind you, that's difficult to finesse even on their own terms inasmuch as John's Gospel often includes many historical details lacking in the Synoptic Gospels. 

One critical test of authenticity is the criterion of embarrassment. And v7 fits the criterion. Indeed, commentators seem a bit embarrassed discussing the verse.

Commentators typically contend that the Peter wasn't totally nude. Rather, he was wearing a loincloth or short tunic. They say this in part on the assumption that Jewish scruples about public nudity would inhibit Peter from fishing in the buff.

However, one problem with that explanation is that, if Peter was already wearing just enough to avoid "indecent exposure," it's less understandable why he'd then don his outer garment. Was he that self-conscious about appearing bare-chested in the presence of Christ? Was that considered unseemly in the hot Palestinian climate? Was that not something Jewish men did? Seems unlikely. 

Richard Bauckham takes issue with the conventional wisdom:

Richard Bauckham However, as you say, everyone went to the [Roman] baths. Even the rabbis went to the baths. I think even Jews, who were more sensitive about nudity than most people, just thought it was natural to be naked around and in water. Fishermen worked naked, even on shore. The Victorians were the first to invent bathing trunks. 

http://larryhurtado.wordpress.com/2014/07/03/paul-and-gentile-circumcision/#comment-10709

Assuming that's correct, it would better explain the account. It's just a bunch of guys here. Like a locker room. And it was so much simpler, when working in water, checking their nets, not to be bogged down by wet clothing. 

There's a certain theological irony about his feeling the need to clothe himself in the presence of his Creator (cf. Jn 1:3). it's not as if God doesn't know what we look like underneath. But people can be (and often are) illogical in that respect. Consider how many Christians still think you ought to dress up for church. Wear your "Sunday best." So it's psychologically realistic. 

Commentators sometimes puzzle over donning an outer garment just before diving into the water. Surely the less you're wearing, the easier it is to swim. Mind you, that objection applies regardless of whether we think Peter was wearing nothing all, or wearing a loincloth or short tunic–before he donned his outer garment and dove in. 

Again, though, that objection misses the point. People will often sacrifice practicality for decorum. Yes, his sense of modesty interferes with swimming, but modesty has nothing to do with efficiency.

So you have this somewhat comical, down-to-earth detail in the midst of an account about the Risen Lord. One of those parenthetical details that lends credence to the account. Not something you'd expect the narrator to invent.