It's a photo target.
In RL, submarines would NEVER have looked into a port to "take pictures" of ships. Recon missions were to take pictures of facilities (phosphate plants/docks), or beach defenses in advance of invasion, etc.
The game engine requires that a unit be a ship to lock it, and it requires a lock to take a picture. Since the game engine does not allow locking land units, there is an invisible object to take the place of what you should be photographing.
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