I want to suggest that Hedgehogs be able to detonate torpedoes.
I think that the explosion from a cluster of hedgehog warheads should be able to prematurely detonate any nearby torpedoes. The explosive shock from the hedgehogs, which are powerful enough to kill subs, should then be enough to trigger the impact pistol on the torpedo setting it off.
For a submarine torpedo, normally, it is best if the torpedo explodes right under, or at the bottom of the ships keel. Most ship's keels are no deeper than 20 meters i think however large battleships can go as far as 40 if im not mistaken.
I know that hedgehogs in real life dont work the way they do in Navy Field (IRL HH's required direct contact with sub to detonate) but since the devs went the way they did with them, I think it is only natural to go with it and explore the possibilities :-).