What is the theme in In the Heart of the Sea: The Tragedy of the Whaleship Essex by Nathaniel Philbrick?

Survival is the central theme in the story.

The narrative of the Essex is, at the end, the story of survival for the men who lived. Of the twenty men who were shipwrecked in the middle of the Pacific Ocean, eight managed to survive despite severe dehydration and starvation. Their tale is one of horror and doing what it took in order to survive. The crew of the Essex chose different ways of trying to survive.