That is a good threory, although on the other hand both Hikari and Kaname are abnormal cases also because they were left wherever to sleep. There might be some epicenter to the phenomenom, and we know there's the border than cannot be passed around the sea town. Were Hikari and Kaname even properly inside of it?

Miuna starting to breath underwater came out of nowhere. It also could be the first time people could move from the land to sea, so it should be quite a significant event.

Based on the preview, did Kaname move to the Kihara household? That might be somewhat awkward. Maybe he's only bunking there for the moment.