Not sure about hydrophones in WWI u-boats, but I think the RN developed hydrophones during the great war and employed them on destroyers and some of their submarines. Perhaps others can provide a more definitive answer.
Nemo
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"I'm afraid there is no disguising the fact that King's obsession with the Pacific and the Battle of Washington cost us dear in the Battle of the Atlantic".
Sir John Slessor GCB, DSO, MC, DL
AOC-in-C Coastal Command RAF
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