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

Survival is a major theme in this book. The sheer hostility of the environment dominates Callahan's every waking moment, providing him with an endless list of chores. The raft must be continually inflated. Water must be harvested from the solar stills. Fish must be harpooned, cleaned and hung out to dry. Should anything fail, break, or become lost, Callahan must be instantly prepared to improvise. When he sleeps, it is the sleep of exhaustion. Callahan's very existence is focused solely on maintaining his own existence.