We all know how destructive the German unlimited submarine warfare was during WW1 and WW2, and also the effective blockade of Japan via the American submarines in the pacific theatre.
Nowadays, things have changed. My point is, the submarine threat seem to be much greater than ever before, due to various changes after WW2.
1: Cargo ships are now HUGE, compared to those in the ww2 era. Oil tankers, Liquified Natural Gas carriers, iron ore carriers, container ships etc. all huge. This means the loss of a single ship would be comparable to the loss of dozens or even hundreds of smaller ww2 era commercial vessels.
sonar much more advanced: the newest British submarine, Astute, can detect surface ship in New York from its base in england. This means modern submarines have GREAT detection range, and since commercial vessels are not designed for stealth, they are loud targets and easily detected by passive sonars.
Submarine’s attack range much greater: modern attack subs are usually armed with anti-ship missiles, and commercial vessles have no ability to intercept nor is it stealthy enough, and they are not designed for survivability either. This makes them extremly vulnerable to missiles.
modern subs more stealthy: subs nowadays are much more difficult to detect and sink, and more costly, because you have to drop so many sonobuoys by airplanes, surface detection is no longer effective because modern subs do not need to surface, and even those that do (diesel-engined) have developed AIP.
modern economy more dependent on trade: this means the importance of trade is greatly elevated and hence its disruption by submarine warfare would be much more serious than before.
I think in a modern war, the sub threat on commercial trade would be so much more profound than before.