What is the main conflict in Adrift: Seventy-Six Days Lost at Sea by Steven Callahan?

In Adrift, Steven Callahan is returning across the Atlantic in the boat he's designed, the Napoleon Solo. His ship is wrecked, and Callahan finds himself alone on the Atlantic Ocean with only a life raft and a limited amount of supplies. He battles hunger and fatigue, fighting his own impulses and stretching out his limited resources, hoping they are enough.