It's a way of foreshadowing the outcome where Moby Dick rams the ship with his mighty forehead. The whale sinks the ship. Moby Dick is the alter ego of Ahab. (You could compare it to Hemingway's "The Old Man and the Sea" where it is a fight between Santiago and the sea.) However in this fable it is about Ahab and the whale. Of course the whale wins. And it takes him, Ahab, down to the bottom of the sea with him. He is swallowed alive by this subconscious force that Moby Dick symbolizes.