The climax of Twenty Thousand Leagues Under the Sea is when the men try to escape the ship.
The climax of a story is the most exciting part, when everything changes.
When the men first join Nemo, they mostly enjoy the adventure even though Nemo seems a bit unstable. They remain his prisoners or his guests until he gets the ship stuck in an ice pack at the South Pole. It is at that point that they start to worry for their safety, especially when they get attacked by a giant squid. Even though we never are entirely sure what happened, we know that Arronax is not going to be looking for any more creatures.