The Submarine is a Modern Era naval raider unit in Civilization VI. It upgrades from the Privateer (or its replacements).
In the Gathering Storm expansion, the Submarine requires 1 
Oil Oil to train and 1 Oil Oil per turn to maintain.
- Attributes:
- Invisible except to City Centers, Encampments, Destroyers, and units in an adjacent tile.
- Can perform Coastal Raids.
- Cannot attack in melee.
Strategy
Unlike the real world, Submarines in Civilization VI are often considered to be a unit with an extremely limited scope of use, and it is most likely to be ignored in gameplay for the following reasons:
- Overall, naval warfare does not carry such importance as it was in Civilization V. The fact that cities now can be settled inland and still enjoy all the coastal benefits, together with the values of coastal tiles compared to land tiles, makes wresting control of the sea low on the priority list. Unless the map is heavily dominated by water, most of the time you can get by even when not owning a single ship.
- Submarines are unlocked at the same time as Battleships, yet are pale in every aspect when compared to Battleships.
- The best selling point of Battleships is that they have a
Range Range of 3, meaning they can reach further inland. In game, 3 tiles away from the coast is the maximum distance that you can settle in land yet still able to reap the benefits of the coast. Meanwhile, Submarines only have a Range Range of 2, meaning tactical settling can render this unit useless. - The Submarines are 2 Movement Moves slower than Battleships (40% slower). When thinking of naval warfare as land warfare if only all tiles were flat, there is a great emphasis on maneuverability and reach, in both of which the Submarines cannot match the Battleships.
- Coastal raiding at this point of the game is not important anymore. Wars in the late game can be damaging towards both the attacker and the defender; if you are about to wage a war, you'd better make it worth. Coastal harassing is definitely not worth it anymore, it is all about capturing and destroying the core infrastructure, and as previously discussed, Submarines do not possess the combat prowess of Battleships.
- Submarines use Oil Oil and Battleships use Coal Coal. Overall, Coal Coal is less valuable than Oil Oil, in terms of availability and the number of applications that each resource can be used for. This is especially important in Gathering Storm.
- The best selling point of Battleships is that they have a
- Invisibility as an ability is nice early in the game, on units with invisibility promotions like Scouts or Warrior Monks, since the enemy does not have a lot of counterplay. However, considering the archenemy of Submarines, the Destroyers, are unlocked right afterwards at Combined Arms, this aspect does not feel too strong anymore. For civs that are far from unlocking Destroyers, they are probably too weak to pose any threats, for civs that are close or already have Destroyers, you cannot do anything about them, so all in all, why bother with Submarines at all?
Note that, unlike in Civilization V, Submarines cannot enter Ice tiles.
Civilopedia entry
When those madcap Confederates in the Hunley sank the USS Housatonic in 1864 AD, it launched a new age of naval warfare; unfortunately, the Hunley also sank itself, taking its crew to the bottom as well. Nevertheless, the submarine had made its stealthy appearance. The submarine, submersible, U-boat is a craft made for sneaking up on a target while underwater and then sinking it with torpedoes … or if possible surfacing and using its pathetic deck-guns. Germany pioneered their use as merchant raiders – really effective ones – during WW1, and expanded upon it during WW2, eventually sinking some 14.5 million tons of enemy shipping (including the odd warship) from 1939 through 1945. Advances in ballast tanks, power plants, control planes, hull plating and such sparked a “submarine-race” between the post-war superpowers culminating in nuclear-powered, deep-diving boats … armed with nuclear missiles that could be launched at population centers. What has the Hunley wrought?
Trivia
- The Submarine's model is based on the American Gato-class submarine.