Since you cannot dive deep to escape, use MOSS (Mobile Submarine Simulators) to lure torpedoes away from your actual position.
To play the South China Sea campaign legally, you should purchase the base game on Steam or GOG . The expansion is automatically included in the latest official updates. If you are using the PLAZA release: Download Cold Waters South China Sea-PLAZA
In shallow waters, being quiet is your only defense. Avoid cavitation (high speeds) at all costs when enemies are near. Since you cannot dive deep to escape, use