Somewhat ironic because Verne was far more pro-British than most of his countrymen (through that might not have been saying much, with The Napoleonic Wars in living memory at the time). And before them, it was the Russians.
Verne wanted it to be the Russians, but his publisher balked at the idea, since France was on good terms with them, at the time. Not only that, but the sales as well. Russia always was a huge market for Verne's books, at one point being the largest of them all, and Hetzel didn't want to lose all that money by offending and alienating the readers. He managed to persuade Verne to change Nemo's nation, and he, ever the pragmaticnote Perhaps surprisingly, but he was. In his youth, when his father withdrew his financial support after learning that his son was dabbling with literature instead of studying the law, Verne had to earn his living by being a stock broker. He hated it with passion, but was quite successful, becoming financially independent even before breaking out in the writing field., agreed.
Ass Pull: Whilst in the Maelstrom, the protagonist writes about how no ship has ever escaped the vortex. He is knocked unconscious, and when he comes to, he and his friends are safe, and offers no explanation as to how that was achieved.
"What happened that night, how the skiff escaped from the Maelstrom’s fearsome eddies, how Ned Land, Conseil, and I got out of that whirlpool, I’m unable to say. But when I regained consciousness, I was lying in a fisherman’s hut on one of the Lofoten Islands. My two companions, safe and sound, were at my bedside clasping my hands."
As mentioned above, the title refers to the distance that the Nautilus travels while under the sea, not the depth that it dives to (20,000 leagues is actually about twice the circumference of the Earth).
Captain Nemo is the antagonist of the novel, not the protagonist. Though he's certainly the most famous character in the novel, he's an Antihero at best, and a full-on villain at worst. This misperception is probably because Aronnax is the Unreliable Narrator who idolizes Nemo before he fully understands what's happening.
The Nautilus crew didn't have an epic showdown with a giant squid, they had a prolonged battle with an entire school of giant squid. The Disney film contributes a lot to this misconception, since (presumably) the studio only had enough money in the budget for one giant animatronic squid.
Related to the above: it's somewhat debatable whether Verne actually meant his famous monsters to be squid; in the original French text, he referred to them as "poulpes" ("octopuses") rather than "calmars" ("squids"), and many early English translations likewise called them "poulps" (an archaic English term for octopi). This may have been in the interest of greater scientific accuracy: in the years since the book was published, zoologists have become mostly certain that Real Life octopi don't actually grow to the gigantic proportions seen in the book, though Real Life squid do.
Crowning Moment of Funny: Conseil hopes Ned Land will get some red meat soon, "lest sir wake up one morning and find only chunks of me to serve him".
Don't forget that he has something of a Misaimed Fandom purely because of his wonderful toys— you read ocean explorers like Robert Ballard and Jacques Cousteau saying that as children, they read the book over and over and "wanted to be Captain Nemo", meaning they wanted his awesome submarine and diving gear, not that they wanted to be supervillains, and two later real-life submarines were named "Nautilus" in homage.
A lot of the descriptions of whaling and fishing and the beauty of the sea becomes this due to modern pollution problems, rampant overfishing, and climate change. Worst of all, some species described in the book (the greater Auk, for example) are outright extinct.
Nemo's not supposed to be completely sympathetic, and this episode seems to be demonstrating the "Extremist" part of Well-Intentioned Extremist. Even Ned Land, a passionate hunter, was shocked at the killing.
There's also all the times they find endangered species, such as a sea otter and a dugong, and kill them. Even when they point out how bad things are going to be if humans don't stop killing and eating endangered animals, they still proceed to do that exact thing with little remorse. And the hunting of a supposed undiscovered species of "giant narwhal" that starts the plot of the book in the first place, as well.
The crew also treat Papuan natives who attack the Nautilus like savages instead of people; they don't even stop one from wandering onto the (electrified) staircase of the ship.
However, also Ned Land, Counseil and Aronnax treat them as savages. Maybe could be said that it was Fair for Its Day? Captain Nemo lampshades that "savages" can be found at any part of the world, and even when the papuans wandered on the electrified staircase, it's stated it was only capable of repelling them and not killing them. Nemo said:
"Well, sir, let them come. I see no reason for hindering them. After all, these Papuans are poor creatures, and I am unwilling that my visit to the island should cost the life of a single one of these wretches."
Designated Hero: Ned Land is kind of a jerkass throughout the film, at least to their "host". Though when we see how bad Nemo really is, he seems a little justified.
Designated Villain: Nemo comes cross as this at first- until you see his true colors. Ned seems more than a little justified in mistrusting him after he sinks a ship full of innocent sailors. This can, in large part, be blamed on James Mason's charismatic performance.
Nightmare Fuel: The sight of the well lit Nautilus speeding towards their ships. Made even scarier when you see the fanatical and hateful glare of Captain Nemo as he rams a ship in one scene.
Older Than They Think: People often forget that there other film adaptations of the book made around fifty years before this one, when the motion picture business was still in its infancy. The earliest one was a silent short film made in 1905, but there were other adaptations made in 1907, 1916, and 1917.
Though it was very nearly Special Effects Failure, as the story on the DVD will attest. The first tests for the final battle took place against a beautiful sunset, on a calm sea. The first Giant Squid had stuffed tentacles held up on wires; these grew heavy and hard to control as they took on water, and the bright sunset made the wires painfully obvious. Disney himself was appalled at this first footage, and demanded a reshoot. This reshoot nearly forced the studio out of business, and the film's nomination for Best Visual Effects at the Academy Awards proved a Crowning Moment of Awesome for Disney.