Rei should just tame a fish or some other sea creature to spot the monster. He's not just an ordinary tamer, after all, he's the inheritor of the legendary beast sorcery master, the wielder of forbidden magic. Not that the author would even remember such a detail from the first chapter anymore. Once located, he can use depth charges to get rid of it.
They forgot to draw the chairs on the bottom panel of page nine lol
Never mind the mangaka, who must be overworked to the point of using poorly photoshopped stock photos for half of the backgrounds just to make the deadlines, but one has to wonder why the publisher is employing the editor if they can't even spot errors like that. At least the mangaka can still provide the chapters, even if by dubious means, but the editor simply isn't doing their work.