- Invisible except when within Sight range of City Centers, Encampments, Destroyers, other naval raiders, or when there is a unit in an adjacent tile.
- Reveals other naval raiders within Sight range.
- Can perform Coastal Raids.
- Has a ranged attack with Range 2.
- Ignores enemy zone of control.
- Does not exert zone of control.
Unlike the real world, the Submarine in Civilization VI has a rather limited scope of use, and will often be ignored throughout the game for the following reasons:
- Overall, naval warfare does not carry such importance as it did in Civilization V. The fact that cities now can be settled inland and still reap the benefits of being coastal cities, together with the value of water 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 without owning a single ship.
- The first naval raider unit, the Privateer, is actually pretty similar to its naval ranged counterpart, the Frigate, since they have very similar stats, except the Privateer having the coastal raid ability. Their respective upgrades, however, are quite different. From this point onward, you can think of Battleship and Missile Cruiser as siege units (they have accessible Promotions that help deal damage to Districts, but low Combat Strength means they are very vulnerable on defense) and Submarine and Nuclear Submarine as ranged units (they have accessible Promotions that help deal damage and outmaneuver other naval units, ignore zone of control, and high melee Combat Strength to resist returning damage). Therefore, Submarines are surprisingly good at killing Battleships and other naval units. The Combat Strength gap is even wider between Nuclear Submarine and Missile Cruiser. This makes Naval Raider units less of "raiders" but more of ranged units and makes Naval Ranged units actually more of siege units. Please note that Naval Ranged units still suffer from 50% damage penalty against Walls and Urban Defenses, but they do not suffer from the extra -17 Ranged Strength, so they are good at attacking cities but they cannot completely replace land siege units.
- Considering wresting control of the sea is not that important anymore, your navy should focus on helping your land army capture coastal cities faster. While Submarines are unlocked at the same time as Battleships, in terms of capturing cities, Submarines pale in comparison.
- The best selling point of Battleships is that they have a Range of 3, meaning they can hit targets farther inland. In game, 3 tiles is the maximum distance that you can settle from the Coast while still being able to build a Harbor. Meanwhile, Submarines only have a Range of 2, meaning tactical settling can render this unit useless.
- Battleships have Promotions that help with sieging, while Submarines do not. With Homing Torpedoes and Wolfpack, Submarines have much greater damage-dealing potential against other naval units than even Battleships with Line of Battle (i.e. two attacks at 85 Ranged Strength vs. one attack at 77 Ranged Strength), though their short Range makes it more difficult and time-consuming to level them up.
- Submarines have 2 less Movement than Battleships (i.e. are 40% slower). When thinking of naval warfare as land warfare waged solely on flat land, there is a great emphasis on maneuverability and reach, in both of which the Submarines are outclassed by the Battleships.
- Coastal raiding is no longer important when Submarines are unlocked. Wars in the late game can be damaging towards both the attacker and the defender; if you're planning to wage a war, you'd better make sure it's worth your while. Capturing opponents' cities and weakening their infrastructure is more important than harassing them and disrupting their economy at this point, and Battleships are better at attacking cities than Submarines.
- In Gathering Storm, Submarines use Oil and Battleships use Coal. Overall, Coal is less valuable than Oil in terms of availability and the number of applications for each resource. In vanilla Civilization VI and Rise and Fall, however, Submarines can be built without resources, giving them at least one advantage over Battleships.
- 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 threat; for civs that are close to unlocking or have unlocked Destroyers, you cannot do anything about them. An interesting note regarding invisibility: if the unit attacks or pillages, it will be revealed until the beginning of the next turn, thus vulnerable to attacks. All in all, invisibility is by no means invincibility.
In the expansions, there is one situation in which you may want to put a higher priority on building Submarines. If you're in a Dark Age and have slotted Letters of Marque, all your naval raider units will have +2 Movement and you'll receive a 100% Production bonus toward training them. This will make your Submarines faster than Destroyers and just as fast as Battleships, and possibly allow you to train enough of them to achieve naval dominance (especially if they earn the Homing Torpedoes, Silent Running, and Wolfpack promotions, which will allow them to severely damage or destroy opponents' ships and retreat before they can react). Once you unlock Telecommunications and upgrade your Submarines to Nuclear Submarines, you'll have an even more pronounced advantage. This is really helpful when you are invaded by a naval powerhouse, since as mentioned above, the purpose of Naval Raiders at this point is to kill as many enemy ships as possible, so they work much better defending since you can control how many Submarines you want by estimating the invading naval power. In offensive missions, sometimes you waste a lot of Production building Submarines when the enemy does not really have a big defending navy, and we already know Submarines are much worse at sieging than Battleships.
Note that, unlike in Civilization V, Submarines cannot enter Ice tiles.
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?
- The Submarine's model is based on the American Gato-class submarine.