The Maunsell Forts are armed towers built in the Thames and Mersey estuaries during the Second World War to help defend the United Kingdom. They were operated as army and navy forts, and named after their designer, Guy Maunsell.
The forts were decommissioned in the late 1950s and later used for other activities including pirate radio broadcasting.
The Maunsell naval forts were built in the Thames estuary and operated by the Royal Navy, to deter and report German air raids following the Thames as a landmark, and prevent attempts to lay mines by aircraft in this important shipping channel. There were four naval forts:
Rough Sands (HM Fort Roughs) (U1) 51.895194°N 1.480569°E
Sunk Head (U2) 51.7773°N 1.50841°E
Tongue Sands (U3) 51.492915°N 1.36662°E
Knock John (U4) 51.562277°N 1.162059°E
This artificial naval installation is similar in some respects to early "fixed" offshore oil platforms. It consisted of a rectangular 168-by-88-foot (51 by 27 m) reinforced concrete pontoon base with a support superstructure of two 60-foot (18 m) tall, 24-foot (7.3 m) diameter hollow reinforced concrete towers, walls roughly 3.5 inches (9 cm) thick; overall weight is estimated to have been approximately 4,500 tons. The twin concrete supporting towers were divided into seven floors, four for crew quarters; the remainder provided dining, operational, and storage areas for several generators, and for fresh water tanks and antiaircraft munitions. There was a steel framework at one end supporting a landing jetty and crane which was used to hoist supplies aboard; the wooden landing stage itself became known as a "dolphin".
The towers were joined above the eventual waterline by a steel platform deck upon which other structures could be added; this became a gun deck, on which an upper deck and a central tower unit were constructed.
QF 3.7 inch anti-aircraft guns were positioned at each end of this main deck, with a further two Bofors 40 mm anti-aircraft guns and the central tower radar installations atop a central living area that contained a galley, medical, and officers quarters.
The design of these concrete structures is equal to a military grade bunker, due to the ends of the stilts, (under water) that are solidly locked into the ground. Many species of fish live near the forts because the forts create cover. They have provided landmark references for shipping. They were laid down in dry dock and assembled as complete units. They were then fitted out—the crews going on board at the same time for familiarization—before being towed out and sunk onto their sand bank positions in 1942.
The naval fort design was the latest of several that Maunsell had devised in response to Admiralty inquiries. Early ideas had considered forts in the English Channel able to take on enemy vessels. 
Royal Marines Crews
For a time the Naval Forts armaments' were manned by Royal Marine's who made up the majority of the crew under RNVR officers and naval ratings in various support roles.
Marines man "island forts" to protect Britain’s east coast shipping.
2 September 1943, in the North Sea. Island forts protect east coast shipping from low flying enemy aircraft.
These forts are commissioned as HM ships and are called after the sands on which they stand. Each consists of two concrete towers, 50 feet high from the base, connected by a steel superstructure on which the anti-aircraft guns and equipment are mounted.
All the Armament is manned by Royal Marines under RNVR officers, with naval ratings for technical duties.
Mail and supplies are regularly delivered to the forts. 
It was reckoned that, during World War Two, the Thames estuary forts shot down 22 German aircraft and around 30 V-1 flying bombs, the Thames estuary Navy forts also destroyed one German E-Boat.
 Wikipedia - Maunsell Forts
 Imperial War Museum Search - Marines man Sea Forts