Severus probably doesn't think he said anything especially funny. Charles laughs anyway. It'd be impossible not to believe in some sort of semblance of ghosts, having seen something like that here, but where the Tranquility is horrifying, Snape's world is whimsical.

If somewhat whimsically horrifying, too. "That's a lot less boring than our takes on things, and I know a few professors it'd shut up. I suppose Smiley's one for the classics, then. For some, anyway, I doubt everyone on the ship has an Ancient Rome."

Yes, he isn't taking this conversation super seriously.