If on, the special graphics for a submarine are used.
Source and more details
The "special graphics" include the periscope circular overlay, periscope direction indicator, depth indicators, underwater overlay, and underwater expanded map.
Left (actual depth) indicator and underwater HUD activation is relative to the center of the PlayerControlObject and can not give readings above water. It requires these property settings:
Right (set depth) indicator depends on a controllable FloatingBundle added to the PlayerControlObject.
If this property is enabled, the soldier cannot exit the vehicle when ObjectTemplate.SetSoldierExitLocation is below water.
Related property: ObjectTemplate.SubmarineData