Traditionally, movies are considered the creation of their directors, that is, since the French New Wave declared the director to be the guiding hand on all movies.
Gore Vidal once said that when he was in Hollywood in the 1950-early 60s, the people who made movies would have thought it hilarious that people believed movies were made , mostly, by directors. It was the producers who made movies.
As in all arts that deal with telling a story, it all begins with the writer. In the case of The Sea Wolf, that was Jack London. In 1904, he wrote The Sea Wolf and it was a bestseller, as were most things written by London. The book told the story of Wolf Larsen, a sea captain who was cruel, intelligent, cunning, violent and crazy. The plot is simple: Larsen's ship, The Ghost, rescues from a shipwreck a dandified writer, Humphrey van Weyden, who is subject to all kinds of cruelties aboard The Ghost. Eventually, van Weyden and another shipwreck victim, a young woman, escape from The Ghost and it is the skills they learned aboard The Ghost that keep them alive.
For the movie, the producers, Warner Bros., hired Robert Rossen to write the screenplay. Rossen re-wrote the story, keeping Larsen and van Weyden pretty much as they are in the book, but re-imagining one of the shipmates into a major character, Leach (John Garfield), and changing the one female in the story, Ruth Brewster (ida Lupino), into a wanted criminal. Instead of van Weyden falling for the girl, Rossen has Leach become her soul mate.