Why submarines cannot replace cruiser diplomacy

How can a submarine provide the diplomatic functions of a cruiser?

It cannot fully do so: submarines can scout and screen, but their stealth prevents them from providing visible presence and socialising roles.

The answer returns to the four S's to distinguish covert military capability from public naval presence. A submarine loses value if it must advertise its position, while a surface ship can deliberately be seen and visited. That difference matters when a government wants reassurance, deterrence, access, or diplomacy rather than a concealed strike option. Submarines can therefore perform only part of the cruiser portfolio even when they are highly capable combatants.