It seems to me that O'Brian doesn't have any particular word in mind when he uses " - ", and that his shift between spelling out the words and using " - " is just a stylistic flourish.
He drops in " - " when the tone is light, as in the example of the girl and the horse, and the scurrilous song sung by the sailors. Under these circumstances he's playing with the literary conventions of the eighteenth and nineteenth centuries for mild comic effect.
But if it's going to interfere with the narrative flow and tension during more serious passages, he just lets his characters swear.

(As for Hemingway ... I'd have to reuse Capote's pithy dismissal of Kerouac, and declare: "That's not writing; that's typing.")

Grant Hutchison