This doesn't directly answer your question, but you may want to check out the YouTube channel "Solo Travel Japan":
https://www.youtube.com/@SoloTravelJapan/videos
They do POV videos of various travels across Japan, and ferries are a particular favorite. There's a lot of videos of overnight ferries across the Inland Sea, but I'm not sure I've seen a daytime one.
The Seto Bridge near Hiroshima has featured in a lot of the overnight ferry videos, so I suspect you might get good early morning sightseeing on some of those as well.