The story of 20,000 Leagues Under The Sea is told in the first person point of view, and also from a retrospective point of view. The narrator is a professor and a naturalist called Pierre Aronnax, and he is narrating a story which has already taken place. You can see that this is the point of view at the beginning of chapter two:

"At the period when these events took place, I had just returned from a scientific research in the disagreeable territory of Nebraska, in the United States."

Jules Verne chose for the story to be narrated from the point of view of a naturalist so that the reader would trust his account of events as reliable. He is, after all, an expert in matters of the sea. Having a retrospective narrator (a narrator telling a story that has already happened) allows Verne to build suspense. Arronax can tell us, in hindsight, that the story he is about to tell is wonderful and strange, which is a good way of hooking a reader into a story.