Submarines

Submarines
Submarines are slow moving units that specializes on sinking larger ships such as Battleships and Carriers, they have the unique ability to submerge to shallow waters granting them immunity to cannon shells, and at deep waters which make them immune to almost all damage excluding the depth charges. At each stage of submersion they loose speed and exhaust oxygen much quicker on exchange for the earlier mentioned immunity, eventually they run out of oxygen which means you have to either surface to replenish the oxygen, depending on the submarine the replenishment time differs yet you don't need to wait for the oxygen to fully replenish.