Yeah, it was the submerged terrain I was worried about. I'll give her the benefit of doubt, even though I don't personally believe it myself that she knows the water there.

But I do believe her to be a local. Now that you mention it, the old grandmother knew about her swimming background while (most of?) the other staff didn't. I wonder how long Nako's been working at the inn (or why she's working there, for that matter). It's hardly one of those casual jobs that students pick up in order to earn a bit of cash.