British ships carried a small detachment of the Royal Marines, compared to the size of the crew. Hood's Royal Marine detachment consisted of four officers (a major, a captain and two lieutenants), 158 men and a boy, from a total crew of 1,400. The King George V class battleships had a larger detachment, with a total of 350 men. The Royal Marines were there to provide ships with a landing party, if necessary, but this was rare during WWII. Hood's Marines fought in Norway in 1940, while the ship was undergoing a refit, while the detachment from the battleship Ramillies made an amphibious landing during the Allied invasion of Madagascar in 1942. As such, most Royal Marines would only see service aboard ship.

In this case, the Marines had several duties, both in and out of action. Where ships needed sentries - the spirit room, the boundary between officer's quarters and the men's accommodation, arms rooms and magazines outside of action - these came from the Royal Marine detachment. They might also form part of the ship's police force, helping to enforce naval law. The Marines also carried out menial work. Officers were entitled to servants, for tasks like laundry and cleaning. These servants were drawn from the Marine detachment. The Marines also provided the ships' butchers and her postal service. Large ships were provided with a Marine band for ceremonial occasions. The band also played a large role in recreation aboard ship, playing popular music for the crew.

In a battle, the main role of the Marines was manning a section of the ship's armament. Traditionally, the Marines operated 'X' turret, the third turret aft of the bow of the ship. The Marines would also operate part of the ship's secondary armament and AA guns. In this role, they operated much as the typical seaman did, moving and loading ammunition, training the turrets and elevating the guns and so on. Other parts of the Marine detachment would be available for damage control if needed. The sentries would remain in position, with the exception of those guarding the magazines. The Marine bandsmen might seem useless in action, but in fact they had one of the most important roles in the ship. They manned the 'transmitting station'. This was the room, buried deep in the hull, which housed the ship's fire control systems. These were complex electromechanical computers, which required considerable finess to use well; bandsmen were seen to have the fine motor skills necessary for effective use of these systems.