The Submarine is the Soviets' primary naval weapon. It remains submerged unless shooting. When submerged, it cannot be seen except if it enters the detection radius of other nearby submarines, missile submarines, gunboats, or destroyers.
The submarine looks very similar to the Missile Sub, but it's not the same unit and visual differences exist.
The submarine's invisibility makes it a useful scouting instrument. It can be placed close to the shoreline, where it would be able to scout on ground units that cannot see it.