According to Ron Howard, to prepare for the role of starving sailors the cast were on a diet of 500-800 calories a day to lose weight.
In the film, Herman Melville himself interviews Nickerson, a survivor of the Essex, and eventually uses his tale as the starting point for his novel Moby Dick. In reality, while it's true that the sinking of the Essex inspired Melville (in particular Owen Chase's account of the events), Nickerson wrote down his memoirs for a minor writer, who never used them. The manuscript surfaced in someone's attic in 1960, and was published in 1984. There is no account of Nickerson ever meeting Melville or interacting with him. Melville did, however, meet George Pollard in 1852.
Tom Holland said that during filming, while on a diet of 500 calories a day, he stole and ate a croissant from the crew catering table. He then vomited because his body was no longer used to the sugar or rich fat.
Due to Tom Holland's age at the time of filming (17), he was not allowed to purposefully lose any more weight than what he had before he was cast, so he put weight on first and lost that instead.
Owen Coffin is renamed Henry in this film to avoid confusion with Owen Chase.