A lot and not much, all at the same time. Clearly, Verne isn't in to the whole tie-it-up-with-a-bow kind of novel closure. Look at what Aronnax writes right at the end of the book:
But what became of the Nautilus? Did it resist the embrace of the Maelstrom? Is Captain Nemo still alive? Is he continuing his terrifying reprisals under the ocean, or did he stop at his last massacre? Will the waves one day wash up the manuscript containing the entire story of his life? Will I finally discover his name?
Will the nationality of the vessel sunk tell us Captain Nemo's own nationality?
I hope so. (2.23.5-6.)
For what it's worth, we hope so too. We've been hoping for chapters and chapters and this is all we get. Kinda makes you want to throw Aronnax back into the Maelstrom, now doesn't it?
This is serious sequel bait and Verne didn't even write a proper sequel. (The Mysterious Island, the real "follow-up," is a total cash-in. The Nemo of that book isn't very much like the Nemo we know. And the Nautilus barely even appears. Lame.)
In the end, Aronnax's parting thoughts give us better insight into his psyche than Nemo's. He writes:
If his destiny is strange, it is also sublime. Do I not understand it myself? Have I not lived ten months of that extra-natural existence? So, to that question which the Book of Ecclesiastes posed 6,000 years ago, "hast thou walked in search of the depth?", two men, amongst all men, now have the right to reply. Captain Nemo and I. (2.26.6)
Now, is it just us, or is Aronnax on a bit of a power trip? He's spent ten months on that sub and now he's getting all Biblical, and acting like he's just as knowledgeable as Nemo… a man who's spent years exploring the depths.
It's here that Aronnax's own skewed perspective becomes painfully obvious. He can't tell us anything about his host, but he thinks he's achieved some kind of enlightenment. It's an ending that makes you want to examine all the observations that have been made before. Aronnax may have seen a lot of great, astonishing things, but you get the feeling that he's missed a lot too.
So we're thinking that's the point of this cliffhanger ending, after all. Similar, in a way, to the conclusion of Catcher in the Rye, as readers, this glimpse of Arronax's ignorance prompts us to wonder what's been missing from the narrative all along—what's real, and what's not.