The trip itself is a bit unnerving; boats are something Niverian doesn't really encounter very much. And when they get to Hades' throne room, he can't help but feel relieved that Persephone isn't present.

He also hates to interrupt the king of the underworld while he's working, but he doesn't want to leave Lindsay to suffer any longer than is strictly necessary, and Hades might know more about why Lindsay was moved. So, he lets out a quiet, polite cough to let Hades know that he's got company.