...and it's available on YouTube, I had to watch it. There's several of them out there, being in public domain, everyone can legally upload it, sell it, modify it, etc.) The ones to see are from "Ark Theater" because the print is beautifully pristine, or "Jackson G", who used software to colorize it.

B&W: https://www.youtube.com/watch?v=wZKisd6qwpA&t=3512s

Colorized: https://www.youtube.com/watch?v=tKRB1ZD7WQc&t=1927s

The good: Innovative (for its time) underwater shots. The length: 8 reels (1 hr, 40mins). The cleanup of the print, which totally outclasses many films near that age. Excellent contrast, no graininess or flickering in the non-underwater footage. The colorized version's underwater footage colored blue (Jackson G) looks good.

But, for a modern audience, there are some truly awful parts. It's actually rather entertaining for the lulz.

It starts off making sense- Professor Aronnax, his daughter, and his assistant are recruited by the US Gov't to help hunt a sea monster. They board the ship, "Abraham Lincoln" and are joined by Ned Land the harpooner. They are thrown overboard when the Nautilus rams the ship. So far, so good.

1. Nemo looks like a thin Santa Claus in brownface because Nemo was an Indian Prince. It's looks bad... truly bad. The actor has white eyeliner drawn under his lower eyelids. There are times when his pupils are washed out and he has scary all-white eyeballs. And what's more pitiful, at 1:13:31, there is a REAL brown person whose skin color and eyes register on camera correctly, which reminds us of how bad the makeup is on Santa/Nemo.
2. The movie is not completely 20,000 Leagues. It intercuts with a different story, inspired by Mysterious Island (but worse). The part about FIVE Civil War soldiers in a (suspiciously 2-D and too translucent) hot air balloon landing on an island is about right. But it goes off on a tangent, with a Jungle Girl observing them, and one of the soldiers, Hero Dude makes friends with her, but one of the others is a creep (with a terrible fake beard) and wants the girl for himself. Ugh.
3. Jungle Girl is another white person in brownface. She's wearing a leopard skin dress, implying that she killed and skinned a leopard and tanned its skin all by herself on the deserted island!
4. There's a whole new non-Verne backstory subplot added: some rich Englishman named Charles Denver wanted Nemo's wife some time ago. Denver grabs at the wife and she kills herself. He steals Nemo's little girl... but we don't know why... he's a pedo? Grooming? Certainly not for ransom!
5. Denver goes back to the Island, because he's haunted by the ghost of Nemo's wife (???). This implies that he dumped the girl there, all alone, years ago (???). He acts really, really weird, like it's too hot or something. He "milks the giant cow" and clutches his chest and faints.
6. The story bounces back to the Nautilus, where Aronnax & Co. (<remember them?) spot a native diver being attacked by a giant octopus (<this is hilarious). Nemo decides to rescue the native and takes quite a bit of time to suit up and we know that realistically, the diver should have drowned. But for the sake of the plot, the rescue is successful and Nemo gifts the diver with a huge pearl.
7. And... getting back to Jungle Girl, NeckBeardie molests her, and the Hero dude saves her and sends NeckBeardie away as an outcast. He bumps into Denver's men and they take him, and unconscious (by heatstroke?) Denver back to the yacht. The outcast gets Denver's men drunk and they go back to the Island, kidnap Jungle Girl and drag her aboard the yacht. Hero dude follows them.
8. Nemo learns that Denver is on the Island and owns the yacht anchored nearby. A-ha!!! it was Denver who caused the death of Nemo's wife and kidnapped his daughter! Now Nemo wants revenge! He has a torpedo loaded and blows up the yacht! Hero dude and Jungle Girl jump overboard and they're rescued by Nemo's sub. When she meets Nemo, of course, he recognizes her as his long-lost child. Nemo's men take a boat to rescue TWO (of the three) men (good guys) still left on the Island but they seem to have lost one of their party- the black guy (???).
9. This should be the ending, but the movie has yet another chapter: "Captain Nemo reveals the tragic secret of his life, which Jules Verne never told." As Nemo relates his tale to ALL his guests, we see Aronnax & Co. once again, because we'd forgotten them already. Nemo's life: India, treasured wife and daughter, meddling Charles Denver, framed for stirring up an Indian rebellion against English colonial oppressors, uprising, arrest, prison, wife's suicide, shots fired, his palace and people destroyed, etc. But reuniting with his daughter was too much joy for Nemo to handle, and he has a heart attack and dies.
10. Nemo is buried at sea, in the coral reefs. Afterwards, the title card says the crew "disbanded" and the Nautilus is left to sink. "The Benediction" shows us the silhouette of two people onboard a ship (prob Hero dude and Jungle Girl) but WTH happened to Professor Aronnax, his daughter, Ned Land and his assistant? Seems they were dropped from the story like hot potatoes, reduced to cameos and never had a chance to develop any personalities? They...didn't ...go down with the Nautilus, did they? This is what happens when the script got too sidetracked with Mysterious Island and Jungle Girl and Nemo's India Drama and forgot about Aronnax and Ned...