Marines might happen they might not there really is not a lot of data available as of yet but I would be fairly certain the ships Master At Arms probably has an oversized department at minimum. Posse commitaitus still applies on the high seas so unless there was a change no Marines. (Note: In the states the military has limited police powers save on thier installations.) A coast guardsman is outside the conventional chain of command due to the service's mission of enforcing the law of the sea and customs law. There are also humanitarian missions that they also preform (disaster relief, search and rescue). Just as importantly they also enfoce maritime saftey and maintain all navigation aids (Buoys, lighthouses). During time of war however marines in small numbers mught be assigned to augment prize crews etc.