Under the discussion of the Super-eels, I indicated that Heuvelmans' idea was that the creature sighted by two naturalists aboard the ship Valhalla was a large eel, one with a continuous dorsal fin starting abruptly some distace from the front, and only the first part of it showing above the water. I cropped an illustration of a conger eel short to show how such a fin might look on such a creature and indicated that doubtless the arrangement would be the simplest explanation for the "finbacked" series of reports.
That would probably be the easiest explanation for other scientists that look into these things. The Valhalla incident is famous for being one of the only Sea-serpent sightings directly viewed by observers (there are also a handful of others, including a couple that make crucial determinations about the nature of the majority of Sea-serpent reports)
However I am not comfortable with that explanation and the main reasons why are provided by the witness Meade-Waldo. He indicated that there was evidence for a larger body under water and he later told Oudemans that he thought the body was Plesiosaur-shaped. One gathers the impression that he inclined toward Oudemans' camp on the subject.
If this was a type of Eel-like fish, the body would have to be unusually long and slender, and the fin on the back higher than the thickness of the body below it. And neither the head or the forepart of the body as indicated looks anything like a large moray eel.
There is a problem in the illustrations to the report as indicated, which evidently follow closely on the sketches of the original witnesses. They are disproportionate, the neck part is too long and the fin part too short given the measurements in the description. I have done a fast pasteup to bring the parts back into something like the correct relative proportions. And after considering the matter long and hard, I still think the proportions are more like the Plesiosaurian category and the "fin" part merely the typical "Hump" part of the back as commonly reported. the "Hump" can be reported as rather steep and with an exaggerated raised sharp edge along the spine. In which cases, the "Back Fin" seems to be a crest continuing the "Mane" of the neck.
This is also not a large creature as measured against other similar reports. The neck is not more than ten feet long and the total length not more than thirty feet. Thirty feet is however the lower end for "Merhorse" sightings as given by Heuvelmans, and this would therefore be a young male just coming into breeding array. I have speculated that the "Manes" in such creatures are temporary, that they are shed and grow back for the next breeding season, owing to the extremely high variability of reported "Manes". Among other things, males seem to rip patches of each other's "Manes" out in duels over females, such that different males have them on different sections of their neck, and usually rather patchy looking rather than continuous (Costello indicates the manes of the males as being on only part of the neck in his composite, but other reports insist on the continuous back-crest or fleshy "fin" from head to tail. And the consensus of opinion is that the "Mane" is fleshy and spiky or bristly, and does not consist of hair but has a consistency more like seaweed.)
The naturalist-witnesses in the Valhalla sighting also said that the creature seen by the Daedalus could easily also be of the same type. That might well be true because that one was also a "Merhorse" of similar colouration pattern and one marked by the appearance of a "Mane" that looked like seaweed.