In What is a Leviathan Doing in Historical Fiction, Part I, it was explained to the reader why such a ‘sea monster’ could in good conscience be considered historical rather than a mere figment of the imagination.
Just because the leviathan in this story is no mere creature of fiction does not also mean any were alive in 1804. Many would confidently declare that all such creatures have been dead for many years, making an encounter with one in the nineteenth century an idea fitting only for science fiction or fantasy. I, however, would contend that their existence at that time entirely possible.
The oceans are enormous, the waters themselves resisting exploration by requiring special equipment to penetrate. Often compared with space as the last frontier, the sea keeps turning up surprises. (Try Blanket Octopus, Yeti Crab, Ninja Lantern Shark, UFO Jellyfish, Sharks in a volcano, Deepstaria Enigmatica jellyfish, Siphonophore, Marianna snailfish, realeased sharks attacked by unknown animal)
The coelacanth is a well-known example. Declared to be long extinct, 65 million years to be more precise, it surprised scientist with its present population, very much alive, off South Africa. That was 1938. (“Coelacanth“. Ocean. Smithsonian Institute. 2018.)
In the last 100 years other large species have been found, such as the megamouth shark. (“Megamouth Sharks“. Oceana.org.)
Now take the giant squid. Fishermen were mocked for years over their reports of enormous squid that were dangerous to man. Then in 1873 the twelve-year-old son of a Newfoundland fisherman brought in the tentacle of a giant squid to a nearby naturalist, Rev. Moses Harvey. The very next month he got a lead to a entire, slain kraken, and so proved their existence to the scientific world.
(Kingdom of the Octopus: The Life-History of the Cephalopoda by Frank W Lane. Published by Jarrolds in London in 1957. Chapter 12, ‘Kraken’, especially pages 186-189.)
Sea Serpents sightings have been reported off Cape Ann, Massachusetts, until the end of the 1800s, and in 1817 and 1818 there were so many sightings the Boston Linnaean Society, an amateur zoologist group, produced a book on the event. (Google Books has digital copies available. It is called: Report of a committee of the Linnaean Society of New England Relative to a Large Marine Animal, supposed to be a Serpent, Seen near Cape Ann, Massachusetts, in August 1917.)
William R Corliss has collected many reports of odd and anomalous things in the world—from ball lightning to ancient model airplanes to huge balls of snakes. In attempt to present true anomalies, not hoaxes, he primarily used reports from scientific journals. In his book, Incredible Life: A Handbook of Biological Mysteries published by The Sourcebook Project in Glen Arm, MD in 1981 he includes an article on “two expert scientists who were actually taking part in a scientific expedition” in 1905 who reported their sighting of a large mystery sea creature with a long neck and a frill behind. (page 519, in chapter 4, “Reptiles and Amphibians”) It is one of several articles he finds worth repeating.
Even land, the dwelling place of humans, is able to hold secrets.
Take the Komodo Dragons. These large land animals—they measure up to 10 feet long and can weigh 300 pounds—were found as recently as 1910 by a Dutch soldier following stories of monsters on Komodo Island. (Komodo Dragons by Jason Glaser, pgs 5 and 7)
The search for more such creatures has not ended. There are actually scientists who specialize in chasing after reports of mysterious creatures. They call themselves cryptozoologists. For reports of current expeditions, try www.genesispark/exhibits. One this website, the Christian cryptozoologist Dave Woetzel not only has accounts of recent investigations on possibly still living dinosaurs, he provides more thorough evidence concerning man’s co-existence with these creatures often considered anciently extinct.
In short, not only do we know that great creatures of the deep have lived, there is good reason to believe they were alive and swimming well into and beyond the year 1804.