Regarding hydrophones, I think the Brits were pretty innovative since they already had this technology in WWI and then later refined it to what came known as the ASDIC or active sonar. If I remember it correctly, it was even Rutherford (the guy who is famous for his nuclear research) who invented the hydrophone. Sounded a bit odd to me in the first place, but that's probably why I remember his name in connection to hydrophones.
Concerning ASDIC however the story is a different one. The British had improved their technology and had ASDIC even when WWII started - however it was still rather ineffective, had some flaws and was then greatly improved during the war - necessity is the mother of invention. However they gave this technology to the US for free when WWII broke out and both countries then conducted further research on that basis. Wouldn't deny though that the US had their own hydrophones by that time, and if not, they then definetly got it from the Brits.
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