One theme is the exploration of the known and unknown world. The other theme has to do with a comparison of the man of intellect versus the man of action.
Take the example of the two characters' behavior on the Abraham Lincoln. Unlike Aronnax, Land never once believes that the mythical sea creature exists. He knows too much about whales. He has the sense to stay aboard the Abraham Lincoln rather than try to harpoon it from a tiny boat. It would have been Captain Farragut's judgment, however, to send Land out to kill the monster on a small boat.