So it actually meant dudes who tended to raising cows? CoolYeah this is pretty much it. Cowboy itself as a word lost a lot of the specificity of its meaning with time, because it originally did pretty much exclusively mean just someone who tended to a ranch's cattle or horses, bu it morphed more into the idea of a character archetype. A cowboy, especially when talking about fictional characters, would now just be someone with personality traits popularly associated with cowboys. Also with space marines, technically military marines are just part of a group of troop that are trained for land or sea. A space marine would just be one of those that operates in space.
Aren't marines infantry trained for amphibious operations?