The Oarfish is a real life bony fish that lives in oceans around the world. Their long, elongated bodies can grow up to 17 meters long. Typically they dwell in the deep ocean at depths up to 1000 meters, but during storms they can be found near the surface or washed up on shore. [ref] http://www.iucnredlist.org/details/190378/0 [/ref] They swim with an undulating motion through the water and feed not on sailors but deep sea shrimp and planktonic crustaceans.
Even in our modern era, we have little footage of the Oarfish in its natural habitat, though Oarfish have been found washed onshore [ref] https://news.nationalgeographic.com/news/2013/10/131022-giant-oarfish-facts-sea-serpents/ [/ref]
a giant oarfish (Regalecus glesne) found on the shore of the Pacific Ocean near San Diego, California. This specimen was 23 ft (7.0 m) long and weighed 300 lb (140 kg). The original photograph can be seen on page 20 of the April 1997 issue of All Hands.
And though oarfish may only prove dangerous to tiny creatures in the deep ocean, their presence is thought to have inspired myths of great sea serpents a thousand times more fearsome than the actual creature.
Reports of sea serpents occur again and again throughout the mythology of different cultures [ref] https://www.britannica.com/topic/sea-serpent [/ref] Similarly as shipping expanded, so too did reports of sea serpents around the world, including nova scotia [ref] http://www.cbc.ca/news/canada/nova-scotia/sea-monster-sightings-off-ns-ebook-1.3314028 [/ref] and on passage from Australia [ref] Strange Creatures of the Sea, The courier-mail 15/08/1934, https://trove.nla.gov.au/newspaper/article/36732071[/ref]
Of course, sighting something in the water from off the side of the ship, any large creature in the water could be difficult to identify from a single glimpse. There are plenty of other seldom seen creatures such as basking sharks that would not be familiar to sailors. Even large clumps of seaweed in the distance might be seen as the massive submerged body of a creature [ref] https://www.britannica.com/topic/sea-serpent [/ref].
Yet even if the Oarfish is not thought of as a fearsome sea serpent, they can still be considered to be harbringers of disaster [ref] https://www.telegraph.co.uk/news/worldnews/asia/japan/7365076/Oarfish-omen-spells-earthquake-disaster-for-Japan.html [/ref]. In Japan, a tradition goes, that the appearance of the deep ocean dwelling Oarfish beached on the shore, or appearing in fishing nets, can mean an earthquake will happen soon.
But all mythology aside, the Oarfish is a fascinating creature, and there are several videos by the gulf serpent project showing Oarfish swimming in deep water.
a king of herrings, giant oarfish (Regalecus glesne) in Ecomare in 20 May 2009, stranded in 19 May. Photo by Salko de Wolf