WW2 "submarines" were for the most part merely submersibles. They couldn't stay underwater for much longer than a day. The batteries would drain out and they would run out of air. It is always best to keep the batteries charged and the air fresh, and this is done by travelling on the surface. Only dive when absolutely necessary - when setting up an attack or when you think you might be attacked by them.
As was pointed out, the hydrophone is purely passive. It contains an array of microphones that listen for the sounds made by enemy ships. It can't give you away.
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“Never do anything you can't take back.”
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