Somehow that didn't surprise me overly much. Kirito's personality suddenly changed between the old show and this, and not in a good way. He can still fight, but the other parts don't make much sense anymore. In the beginning of SAO he was the lone wolf and a beater, then he slowly began to socialize with Asuna (and Yui), but his relationship with others was cordial, though he took a supportive attitude toward some of the girls. However, he still did what he wanted. Now, he's clinging to Sinon like some sticky n00b, as if he couldn't spend a moment alone. It's understandable he chose to gain information from her in the beginning since he has no particular love for the game, especially when he for some unfathomable reason didn't even google GGO to find out anything about it, but now later it's exceedingly strange. The main reason is what I mention above: He's basically on a dangerous mission here, so by hanging around with Sinon constantly he's in fact subjecting her to extra danger as well, all the more because he's incapable of coming up with any other nick for his character. I imagine he's going to leave the game to go back to his harem when he's finished with Death Gun, so what good is making friends here?

In short, his other actions make so little sense that taking an unnecessarily naked dip is to be expected.