Britannian Navy

While battleships are seen used by the Empire in early battles, ships do not seems to play an important military role in the Empire, usually delegated the role of transport and trade. This is possibly why the Black Knights chose a submarine as one of their first headquarters. However, the Britannian military is by no means incapable of aquatic or even deep-sea combat, as shown by effectiveness of the Royal Marine Infantry (RMI)-13 Portman series of amphibious frames.

Britannian Marine
Not much is known about the Britannian Marine other than they serve as a branch for the navy, it is safe to say that the role of the Marines in the Code Geass universe is still the same as it were in the real world they are responsible for the use of Amphibious Warfare during missions and serving as the Ground Units for the Navy, they are only shown in screenshot during the first episode performing amphibious landing on the shore of Japan. it is possible that the Britannian Marine have it's roots from the former Continental Marines led by the former General Washington during the Washington Rebellion but currently without any evidence this is nothing more than a speculation but if it were true that mean the Britannian Marine is the Code Geass equivalent to the United States Marine Corps

Royal Marine Infantry (RMI)
The Royal Marine Infantry or RMI is the official designation for the Knightmare Corps. in the service of the Britannian Navy the RMI specialize in Naval and Amphibious Warfare although during Battle of Mt.Fuji a single unit of Portman II equipped with Float System can now participate in Aerial Combat but to what extend is not yet made clear, the Mechanized division is the equivalent of the Royal Panzer Infantry of the Britannian Army unlike the Royal Panzer Infantry the RMI receives little notice in the series and only appear in a few episode. In Area 11 the Navy is seen to be stationed in Shikine Island along side with the RMI protecting the base from any Naval threat. The RMI staple Knightmare Frame is the Portman series.

Warships
Two types of Britannian warship are shown. The most common are the Destroyers, which may have been modelled on the real-world Type 45 Destroyer. No class-name or ship names are given. Its armament includes a deck-mounted cannon and a vertical missile system, which can fire ASROC, though it may include missiles of other types. The other type is a larger Carrier-Battleship hybrid (search the Gallery). It is shown as having a trimaran hull, with the outboard hulls supporting two flight decks. It's armament includes two twin gun turrets, a vertical missile system, and numerous CIWS turrets, along with a complement of Portman knightmares. No names are given.

Their main appearance is in R2 Turn 07 The Abandoned Mask (episode) which Suzaku attempts to chase down the Black Knights submarine with the help of a naval task force consisting of one Carrier-Battleship and at least 7-8 Destroyers.