-to goldorak-
On the one hand is not required to make the game as difficult as it is possible. Do not forget that it must be playable
Realistic not always means difficult.
If torpedo is visible on the sonar screen and not reported it's not reallistic. For example, I can be on TMA (making solution) and I can miss the torpedo launch event. Modern sub will be killed because is blind...
Torpedos are too fast to let them a chance being unspoted. And this is not possible to be all the time on sonar station because working on TMA is the key to win (as you wrote).