An interesting book. It has tremendous strengths and tremendous weaknesses. It is clearly hard science fiction. I think that this is not obvious to many people because so many of its predictions, such as the electric light, have come true. Like many hard science fiction books, it is also an adventure story.
The book is fascinating, because you can see the hard SF genre being invented. It also has a great, but underutilized character in Captain Nemo. The submarine is great and some of the incidents. The rest of it is a problem.
Verne's prose is bad. He has a tendency to rhyme off facts in an intrinsically uninteresting way. When he is talking about fish, which he does often, he usually lists them with their biological classification. This is amazingly uninteresting. He will also recount stories and bits of travelogue in a profoundly boring way. He doesn't really fill you with a sense of wonder. He is in love with science and technology, and he fetishizes numbers. That could be done well, but it isn't. He also loses opportunities with the adventure elements. He manages to make finding the lost continent of Atlantis quite dull.
Once the giant squid appears, which is almost at the end, it picks up considerably. The last 60 or 70 pages are what the rest of the book should have been. Worth reading, despite the fact that it can be frustratingly boring at times.