Septimus does not even notice anything remotely like arrogance. Instead his eyes just brighten with fascination and excitement. "How do you deal with the local ambient moisture not actually being infinite in supply?" he asks eagerly. "Are you able to pull that much from that far away?"

See, the thing is, Septimus knows about that trick, though he's not seen even the more powerful watercrafters be able to pull it on quite that scale and sustained rapidity. The latter, perhaps, any child of the High Families could do. He's never tried it himself, but he'd guess he could at least sustain the job for relatively long periods if he had to, though perhaps not quite as efficiently as Skulduggery seems to--and he has no doubt his father could. But then, he's used to furycrafters who can do that, who can face down earthquakes and volcanoes and hurricans, freeze floodplains... No. It might be somewhat arrogant, but as far as the Princeps and one of the most powerful scions of the House of Gaius is concerned? This is just par for the course, really.

So all he displays in response is absent acceptance and simply focusing on how very unexpectedly useful it is. And there's little arrogance in fact.

Though perhaps some of that resides in Septimus's own arrogance and overconfidence.