20,000 Leagues Under the Sea
Well, let's see… ah, yes, here it is:
I am going to embark on a new underwater tour of the world—who knows, perhaps the last?—and revisit everything I have studied on my many travels; and you will be my study companion. (1.10.48)
So, Nemo is taking Aronnax on a tour. And you know what kinds of people love giving tours? Guides. There you have it.