Submarines were naval vessels designed to travel underwater for a number of different roles. Both the Coalition of Ordered Governments Navy and the Union of Independent Republics navy maintained their own submarines with the Coalitions' submarines tending to be larger than the UIR submarines. Some NCOG submarines are known to be as long than a Thrashball field. 
Submarines are underwater-going vessels designed to survive and operate under the surface of the water. These naval vessels vary in size from miniature submarines with a crew of a few sailors to titanic, Thrashball field-length naval vessels with a much larger crew.  Submarines were armed with torpedoes, with some also being armed with Flechette Guns such as the Seabass Model. Some submarines were the actual weapon, such as the UIR miniature submarine that was used as suicide vessels. 
During the fifteenth year of the Locust War, as the remnant of the COG were living on Vectes, they captured a man named Massy. Colonel Hoffman found that he was the third man who raped Bernie Mataki. So, they brought him into custody. Instead of a fair trial, he would be executed. However later, a deal was struck with the Stranded group the Massy was with. The COG would hand over Massy, and the two groups would forever leave each other alone. However, the COG thought up another plan. Before handing over Massy, the Submarine that they brought along with, would fire at the Stranded ships, destroying them. Only up to two small ships were allowed, but the COG brought along the submarine, the CNV Clement, because of the cability of Submarines to be undetected while underwater. After the trading deal on the sea between the Stranded and the COG, who were handing over Massy, a little surprise happened. The remnant of Gorasini appeared, with a Submarine of their own, The Zephyr. Commander Miran Trescu, who was also the leader of the remnant of Gorasni, had wanted a deal with the COG leader, Chairman Prescott. After meeting with him a deal was made, in which the remnant of the Gorasini, which was about 4,000 civilians, would be allowed to land on Vectes and establish their own base there. In turn, the Gorasni would join the COG. And, the remnant of Gorasni would supply some of their fuel to the COG, and the COG would help to protect the Gorasni base; as because the remant of Gorasini had now joined the COG.
The subs Zephyr and CNV Clement defend Vectes from attack by Lambent Leviathans during the First Battle of New Jacinto and the Clement runs out of torpedoes and is severely damaged. Later, the Gorasni leave and take the Zephyr with them and its commented that the Clement will probably have to be retired and it apparently is.
After learning of Azura and Adam Fenix's weapon there, Delta Squad and Dizzy Wallin travel to Endeavor Naval Shipyard to retrieve a submarine and travel to the island as it is the only way of getting past its Maelstrom barrier. The group finds one sub still intact in a maintenance bay and have to find a new rotor and fuel for it. They succeed and Dizzy proceeds to pilot it to Azura through a sunken city, a minefield, attacks by Manglers and a Leviathan, a strong current from the Malestrom and torpedo attacks from automated launchers with Delta manning Flechette Guns to defend it. The sub reaches Azura, passing through a field of dead bodies from civilians slaughtered by the Locust before finally docking on Azura. Delta disembarks and has Dizzy leave to a safe distance with the sub as it may be the only way off the island. After the battle, Dizzy docks the submarine and comes ashore.