I've taken a few of those Baltic cruiseferries, but it was ages ago, so probably not that one. They're pretty much what you get if you cross a cruise ship with a roll-on, roll-off ferry. It is, however, absolutely a ferry. And even if it's one of the largest ferries that exist, there are some that are significantly larger.
Not yours, the one in the manga
The one depicted here is not as large as the Baltic one (although I wouldn't be surprised if the art is off by a lot), and it looks like the cargo hold is much bigger relative to the size of the ship, so having a much lower passenger capacity makes sense. It's also still a ferry.
Another example of the gaps in the author's knowledge would be regarding the prefix for a vessel's name, for example a ferry of that nature would have prefix of MV (motor vessel) rather than SS (steam ship) as used by the author.
I'd chalk that down to the character not knowing the difference, even if it's a generous interpretation.
Whenever I see Norse runes in manga (or anything else), I check if they're readable. If they are, I translate them. Most of the time when manga has readable runes, they're in English, but I've also seen Latin and some other languages. /tangent
