The full set of pictures makes this even more bizarre. He's quickly putting shorts on in front of a father and a young child, as Katy wonders off.
It seems really odd to be naked. Especially when he was previously paddle boarding in shorts, and then seems to have come into shore, stripped off and gone out again, leaving his shorts on the beach.
There's probably some truth in the speculation that this was good PR for them both. That or he has uncontrollable naked urges.