He feels the slight pressure, if not quite on the eardrums he doesn't have. That was one of the most difficult things Skulduggery needed to adjust to after he came back from the dead; his senses all existed, but sometimes in very different places than he was used to them being. The change in air pressure is enough to convince him, even if Septimus's next question hadn't already.

"I'm a sorcerer," he explains, as though the phrase is a normal part of an introduction - which it is, both for him and very likely for Septimus as well. "An Elemental, to be specific. I can't purify the existing water supply, but I can add to it fairly quickly."

As a demonstration, he takes off his hat and clenches his hand above the brim. Within seconds, it's filled with sparkling clean water, like a miniature volcanic lake.