In the interest of accurate reporting, it should be mentioned that one of the boats has been connected to a unit of North Korea's army, along with Kim Jong-un's apparent insistence on fishing as a source of food and foreign income. So the leading boring theory is that these are North Korean ships, risking literal life and limb in order to catch a mackerel or six for the Great Leader.
Wait, hold on. That's ... actually even more terrifying than a dark saltwater god stealing fishermen's faces or whatever. Imagine that your entire lot in life is sailing notoriously stormy and awful seas in a barely equipped vessel, only for your crew to face the unspeakable horrors of the ocean. Maybe things get so bad that you end up with a Donner Party situation. Finally, after the inevitable gory climax, you wash up in a foreign land, where your badly decomposed mortal remains are collected and cremated by stoic Japanese coast guards who have at this point seen way too much of this shit to give a damn.
Around Act Two of that story, having your soul eaten by a horde of ravenous ocean witches would probably be a welcome respite.