The implosion is done in two nanoseconds, followed by a large explosion, which would be picked up by our sonar nets listening for Submarine traffic.
All the contents of the sub - oxygen, flesh, etc - are compressed by 320 atmospheres in two nanoseconds. The heat caused by compression - boom. Navy absolutely heard that, unless they were all in class learning queer theory.
The nanosecond claim is preposterous. Light travels 10 inches in a nanosecond. Water has issues with such speeds.