Actually not so impossible
There were a few recorded instances of sub vs sub action - submerged. One case was towards the end of the war when a Brit detected, fired on and sank a submerged U-Boat. This was obviously very rare but in fact did happen. How they did it was to plot bearings over a course of time and developed a solution much like today's TMA techniques. Using a spread of six fiish solved for inaccuracies in the solution. What was hard though was guessing the depth, which the Brits did manage to do.
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