[the dark knight]

It uses a sonar ping to determine depth under your keel. In real life, yes, this ping would give you away to the enemy. In SHIII, the enemy ignores the depth ping. What I do, (for realism for myself) I will not ping when I am near a convoy, or an enemy warship. Merchants did not have hydrophones, so you can be as noisy with them as you want, as long as a warship is not near.