I mean...if it's sunset on the water, I would expect it to be somewhere on the southern side of Honshu. If it's an island in the Sea of Japan right off the coast of Honshu, then you'd probably see the sun setting on the land of Honshu since it's so close. Like, that doesn't really make all that much sense to me given what we see on the map.
Whatever. It's ultimately not a big issue. The bigger issue is that this series has gone on for too long and makes little sense at this point.