Submarines dive underwater, and amphbots, Iris, Aspis, and certain striders are amphibious and become submerged to some degree when floating in water or walking on the seabed. Certain buildings when built in the water are built on the seabed. Fully submerged units can be difficult to view, detect and attack.
When a unit or building is fully submerged, the unit cannot be seen by most units, and cannot be detected by radar; instead, a sonar is needed to detect and see the unit. It further receives protection from most types of weapons. The units and buildings that are described as "Undersea fire" or "Anti-sub" generally include at least one weapon that can fire at fully submerged units. Weapons that can fire at targets above water are described as "water capable" if they can also shoot at fully submerged units. Torpedo weapons, being able to target only units in the water, do not possess this description. Many (but not all) units with "water capable" weapons have sonar.
Normally, submarines are always fully submerged. However, a submarine that gets very close to a beach or otherwise somehow gets up on land and is thus no longer fully submerged can be attacked by units that otherwise can shoot only at surface targets.
All amphbots regenerate health while they are floating or submerged in water. Many amphbots can also float to the surface instead of standing on the seabed, which is usually necessary for them to be able to attack.
Submarines only require a relatively shallow depth of water to be fully submerged, while amphbots, being generally taller, require a deeper depth to be fully submerged.
On maps with deep water, it is possible to sneak fully submerged units under opposing units if the opponent's units and buildings do not possess sonar or anti-sub weapons. This can be especially dangerous with units such as the tactical nuke missile launching Scylla and the teleport-enabling Djinn. Packs of attack submarine Seawolfs can cause great damage to units and buildings in the water while taking little or no damage in return if the opponent has not invested sufficiently in anti-sub units and buildings.
Defending against submerged units (anti-sub)
All amphbots, ships, submarines and Hovercraft Platform possess sonar, while multiple amphbots, ships and hovercraft have anti-sub weapons. Urchin is the basic defensive building against submerged units, possessing both a torpedo weapon and a sonar.
Flying units that are useful against fully submerged units and buildings are limited to the Raven, the Owl and the Athena. Both the Owl and Athena do not possess weapons, but do possess sonar. The Raven, on the other hand, does not possess sonar, but its bombs are anti-sub. Combining Ravens with either an Owl or an Athena can therefore create a very mobile force for attacking fully submerged units and buildings.
Other anti-sub units and buildings of note include:
- The Gauss defensive building, which is more of a coastline defensive building than anti-sub for two reasons: First, it cannot be built in the water, only on land; Second, it does not possess sonar. Its weapon is still "water capable" and thus can attack fully submerged units.
- Both the Ultimatum and the Detriment have sonar and have one weapon that is "water capable".