Intrepid's Marine Datachment performed as security (an internal police force) and had
certain duties under combat conditions. They also served during specialized assignments in communications, logistics and other positions.
Marine Detachment
In today's moderan Navy, almost all naval vessels have Marines on board. When assigned Marines do embark and are not generally assigned to or stationed on the carrier for a full tour. In most cases, the Marines are part of the assigned security Marine Detachment or ( MARDET) or sometimes embarked with a squadron of Marine Corps fighter aircraft. Some of these squadrons can include a mix of both active duty and activated reservists from the Navy and Marines.
Marines assigned to Marine Detachments aboard carriers are not classified as part of the ship's mission personnel. They spend most of their day-to-day time in training (combat skills, physical conditioning, and training/education/exercises related to the MarDet's mission) and doing required ship's training and drills for functions where they are integrated into the ship's crew for either emergencies or combat (gun crew, damage control/firefighting, defense against boarders etc.) Naval vessels that don't have Marines on board are typically very special purpose vessels, like some submarines, research vessels, or something like a minesweeper.
In certain cases Marine expeditionary and amphibious units are embarked on the so-called "Gator Navy", the amphibious warfare ships that carry ~1500 to 2000 combat Marines. These expeditionary/amphibious units are deployed on Amphibious Assault Ships (LHA/LHD), Amphibious Transport Dock ships (LPD), and smaller landing ships, which have various capabilities for putting landing forces ashore by air or across the beaches. These combat Marines are deployed on the ships, but are not really part of the ship's complement (crew); The combat Marines are carried by the ships, somewhat analogous to weapons to strike an enemy. ​