Naturalism is a sub-genre of realism. Crane's writing reflects realistic and naturalistic qualities. The Open Boat is an example of realism because it portrays events objectively and realistically. The narrator does not cast judgment but explains events as they happened in order for the reader to experience them without a type of moral bias.
This piece is also an example of naturalism because most of the characters choices are beyond their control; they are in the control of nature. The main theme of naturalists were whether humans had control over their own fate, and The Open Boat is primarily concerning this theme.