Submerged is the state of a unit that is completely under water and, subsequently, undetectable by units above the surface, even if within their line of sight.
Any unit that submerged cannot be detected by radar and is subsequently rendered stealth. Additionally, units that are submerged cannot be targeted by any weapons other than torpedoes and Harmonic Depth Charges, rendering them immune to most weapons.
This comes at a cost, however, as only torpedo and tactical missiles will work under water, and if a unit is unequipped with this type of weapon, it will be helpless to defend itself against units with these weapons.
All units in the water, whether on the surface or submerged will be detected by sonar unless under the cover of sonar stealth.
Among all units that can be submerged, some are able to dive.
Slivers cannot be attacked by Trident Class units when submerged, but retain the ability to attack using their Nanite Torpedo launchers.