The best place to look for this information is the chapter that is titled "At Full Steam." The previous chapters do make mention of the sea monster, but the narrator is mainly relaying rumors about the monster's supposed size and looks. It is even in question as to whether or not the monster is real or not. It is not until Ned Land shouts out that he sees the creature that readers get a solid description of what it looks like.

Ahoy! There's the thing in question, abreast of us to leeward!

The first thing that Aronnax points out to readers is that the monster produces light. He specifically mentions that it is not in the form of common bioluminescence. Next, readers are told that the monster has electric capabilities.

No, sir, it's apparently a gigantic narwhal, and an electric one to boot.

Aronnax is not sure about the narwhal part, but what is very clear is that the creature is fast. Their ship is built for speed, but even though Aronnax and the others on the ship are trying to run away in this chapter, the creature still comes at them at a speed double their best speed. Eventually, daylight returns, and readers are told that the creature has a "long blackish body" of about 250 feet in length. Finally, the "monster's" skin is hard enough to bounce a cannon ball off of it.