Like mwestwood, I would point to the spotting of Moby Dick as the climax of the novel. We can describe this moment as the "turning point" also if we want to say that the turning point occurs when the conflicts of the novel are fully exposed and poised to be resolved. We have turned the corner of exposition and arrived at the moment of complete exposition when the plot lines are all laid bare, so to speak.
In Chapter 36 after Ahab finally emerges and comes on deck, he declares the mission of the Pequod: to find and kill Moby Dick. Then, after a sprawling narrative, in Chapter 133 Moby Dick is finally spotted. This is the climax as the men prepare to harpoon this white monster. When Ahab is caught in the harpoon ropes is probably the highest moment.