The Navy simply does not have enough attack submarines, cruisers, destroyers or helicopters to adequately protect the its deployed forces [aircraft carrier groups and other surface ships] from subsurface threats because its ASW capability has atrophied since the end of the Cold War.
The Navy will need a new platform to replace the retired S-3 Viking.
One option would be to bring the S-3 Viking back into service.
The Navy could also develop a follow-on S-4 — in a manned/unmanned or straight unmanned configuration. Or, the service could opt to build a follow-on to the developmental MQ-25A Stringray unmanned aerial refueler that could serve as an ASW platform.
Hendrix noted that during the battle of Midway, the storied carrier USS Yorktown was finally finished off by Japanese submarine-launched torpedoes. Thus, while anti-ship ballistic and cruise missiles often garner the lion’s share of the attention, some of the deadliest threats faced by the carrier come from under the sea in the form of torpedoes.
Particularly troublesome are Russian-designed wake-homing torpedoes that are generally immune to most counter-measures — and are often an overlooked threat.
Moscow has proliferated those weapons to any nation that can afford to purchase a Kilo-class diesel-electric submarine.
Kilo class submarines can be purchased for about $200-250 million
SOURCE - National Interest, wikipedia