We are very focused on defending against threats from the air, A2AD and all that jazz, and for those below, we have nifty little countermeasures and anti-torpedo defenses we are trying out - but those are focused on what we know and am comfortable with - traditional torpedoes.
Via The Economist, I'm not quite sure we are that ready for this;
WHEN introduced 40 years ago, the Soviet Shkval (“Squall”) torpedo was hailed as an “aircraft-carrier killer” because its speed, more than 370kph (200 knots), was four times that of any American rival. The claim was premature. Problems with its design meant Shkval turned out to be less threatening than hoped (or, from a NATO point of view, less dangerous than feared), even though it is still made and deployed. But supercavitation, the principle upon which its speed depends, has continued to intrigue torpedo designers. Now, noises coming out of the Soviet Union’s successor, Russia, are leading some in the West to worry that the country’s engineers have cracked it.That means you need to kill the archer if you can't do anything against the arrow.
In October 2016 plans emerged for a new supercavitating torpedo, Khishchnik (“Predator”). Few details have been released, except that the work is being carried out by Elektropribor, a design bureau specialising in high-precision systems for submarines. Combining a General Dynamics-style sonar with a better motor could, however, result in a weapon that the world’s navies would truly have to fear.
Such a motor is possible, according to Georgiy Savchenko of the Institute of Hydromechanics at Ukraine’s National Academy of Sciences. His supercavitation-research group estimates that with the right fuel (perhaps lithium, which packs more energy per kilogram than magnesium) a new torpedo could have ten times the range of Shkval. It would still be noisy, but, added to its speed, such a combination of range and tracking ability would make it hard to evade. Moreover, there is no theoretical reason why Khishchnik should not travel quite a lot faster than Shkval does. In laboratory tests, supercavitating projectiles have clocked more than 5,000kph.
Yea, about that ...