Overnight ferries around the mainland

Hi,
Last year I went to Osaka ATC building and while eating at a restaurant there I watched the overnight Sunflower Ferry take off for Beppu. Looked quite cozy as it disappeared into the night.

I actually went from Takamatsu to Kobe by ferry, which was quite a relaxing experience and the ferry was not too full so I could lie down in the sofa in the entry deck and enjoy the view. The noodles were fine as well.

I noticed I got a bit seasick but it could have been because of lack of sleep too, not sure.

Anyway. What are the overnight ferries like?

I saw there are different bed types, like tourist bed sleeping on the floor, or dormitory style with bunk-beds of 8 or 4 in a room, or single rooms.

I think the single room was around 17000 JPY from Osaka to Beppu. I suppose they are even more on the long distance ferries from Hokkaido to Honshu or so.

Luggage is no problem, only a backpack, but I would like to lock it in somewhere if sharing space as I have valubles in it.

Is it any interesting experience in doing overnight ferries?

Are there any parts around Japan that are more prone to rough sea than others? I supposed the Seto Inland Sea would be calm, so if it is too rough, maybe it would be a problem... Not sure.