Like King Ahab in the Bible he is seduced by a false god, in this case the god of vengeance. His disobedience causes him, like Jonah, to be swallowed by the supernatural fish. But whereas Jonah always acknowledged God, Ahab does not, and he is not forgiven. The fish does not vomit him out. Instead, Ahab dies.
Whales are mammals.
They breath air and they are warm blooded. Their bodies are hairless and streamlined for fast swimming. They have a large layer of fat which also streamlines their bodies and keeps them warm in cold ocean water. Moby Dick is the whaler's nightmare. Thousands upon thousands of these giant animals have been killed without turning on the boats, the ships and the sailors who were killing them.