[quote=Wernher]
Well, submarines can destroy other submarines, but still, can we have multiple ships?
[/quote]
Submarines are best at Anti Surface Warfare and Strategic Missile launches. Yes, they can hunt and engage other submerged submarines, but it is not their primary role. They are ship killers. Destroyers and Frigates serve the role of hunting and killing submarines more efficiently. Besides, submerged boats are slower than surface ships.