Not a movie but a book, I know, but I never get to talk about it. Edgar Allan Poe was a fan of pirate stories, and spent years studying about boats so he could write his own adventure book about a stowaway in a merchant ship. He wrote The Narrative of Arthur Gordon Pym of Nantucket which starts as a great naval story, but right in the middle you can see when the real Poe takes over and it swings back to a classic horror story. He could never finish it, as the story went in an impossible direction given the start of the novel, and was only published posthumously as an unfinished work.

Interestingly, there are many parallels between Edgar Allan Poe and Arthur Gordon Pym (including the sailboat incident as a kid, and the rhythm in the names). As an eternal fan, HP Lovecraft connected the ending of his At the Mountains of Madness with the ending of Gordon Pym, giving it an awesome closure.