I don't know i mean.
The Major Thing probsbly would be a good sound Engine which simulate real sound waves in the Water which are faster and slower depending on the water (temperature, Salt percentage) and the Freqiemcy.
Properly working Broadband, Narrowband, Demon, SSP, different ARRYS depending on the Sub. And apropiate ASW weapons like ASW Equipment for Jets or Planes like Torpedos or buoyancy,
This maybe look alot but keep in mind most of this calculation cxan be reviewd in Dangerous waters and Used from there.
The rest would be normal things like rocket lanches, 3D Interior etc.