[quote=Gunther]
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.
[/quote]

That logic is valid for ships that use batteries to power their engines when submerged (ie U-Boat VII and variants without a Snorkel) where the batteries have less output than the diesels when surfaced. 

However, in modern times, where submarines are powered by engines that can run in both states (surfaced and submerged) they either loose a few knots, or even go faster submerged than surfaced. As well as being engineered to be hydrodynamic when submerged