Twenty Thousand Leagues is science fiction in the sense that it is a work of fiction that is loaded with scientific details. Verne must have done a tremendous amount of research to prepare for this novel. He supplies so many classification details about sea life and underwater plant life and so many technical details about the oceans, seas, and currents that sometimes it starts to bog down the narrative. Verne was a great author of adventure stories, but if you want to read Twenty Thousand Leagues as an adventure you can read the first quarter of the novel and then skip to the last quarter. The middle section is a long scientific expedition, but when the action starts up again it is exciting.
The edition I read is an interesting one. Published in 1993 by the Naval Institute Press, the translator states that he restored a quarter of the 1870 French text that had been omitted from previous English translations. He says older editions emphasized the adventure aspects of the novel and edited out a lot of the scientific detail. As stated above, I have mixed feelings about the restoration. Rather than a hack translation, it may have represented some judicious editing of a text that kind of drags in the middle.