The story takes place at sea, starting specifically in the Pacific Ocean, where our somewhat weak protagonist Humphrey van Weyden is aboard a San Francisco ferry. When the ferry crashes, van Weyden is picked up by a fishing schooner, the Ghost. It is aboard the Ghost that the "city boy" van Weyden learns to be strong. London portrays that idea of strength as being able to survive in nature, in the elements. Van Weyden learns skills of hunting and survival which he shows mastery of when he ends up on the deserted seal island.
Wolf Larsen, the captain of the Ghost, represents a naturalistic survivalist - consider his chosen first name. However, he is also a sociopath. He has taken the idea of naturalism to an extreme that has caused him to neglect any concern for other human beings.
London uses this setting, and these characters, to show the drawbacks of American industrialism. The hierarchy on the Ghost mimics the hierarchy in business, and Wolf's bullying behavior shows the ruthless nature of businessmen. London is also showing the balance that must be struck up between civilization and nature - he was trying to explain to a country moving away from nature that we are still connected, and we must honor and respect the forces around us.