Pierre Aronnax is an academic to the core—a true expert, a real authority on all things oceanic. So, on the one hand, you might expect him to write kind of dully and stiffly. But you can't really expect the guy to write without any emotion when he's surrounded by totally mind-blowing sea stuff.
That'd be like asking the world's foremost candy expert to stay cool in Willy Wonka's chocolate factory. It's just not possible. So while there are some long, boring passages about various fish, about the way Nemo's ship is powered, and so on, in this book, Aronnax—and Verne—take the time to gush about the surrounding wonders.
Salt and pepper Aronnax's tone with Nemo's air of mystery and menace, and you've got a book whose tone can shift pretty rapidly. One second, Aronnax may be listing a bunch of swimmy sea creatures, and the next, his mind may be racing with questions about Nemo's origins.
This mixed bag o' characters allows Verne to keep 20,000 Leagues interesting… and, with the help of Ned and Conseil, just a little bit funny.