A hedgehog hit should be near catastrophic to a submarine. Two would usually "kill" it. I know this site is not the most reliable, but there are other sources:
Wikipedia: Hedgehog (weapon)
toward the bottom 1/3 of the page:
4. A direct hit by one or two Hedgehog bombs was usually sufficient to sink a submarine.[citation needed]
Many depth charges were required to inflict enough cumulative damage to sink a submarine; even then, many U-boats survived hundreds of detonations over a period of many hours—678 depth charges were dropped against U-427 in April 1945. The depth charge, usually exploding at a distance from the submarine, had a cushion of water between it and the target which rapidly dissipated the explosive shock. The Hedgehog's charge, on the other hand, exploded in direct contact with the hull. However, near misses with the Hedgehog were silent and did not cause any damage, unlike the cumulative damage caused by several depth charge near-misses; nor did it have the same psychological effect as a depth charge attack.
Who do you inquire of about that 'citation' detail?? I also wonder who they asked about that "...nor did it have the same psychological effect as a depth charge attack"... quote?... lol - In the game, you don't hear a drop as they're passing over, so you ~know~ they are up to something - but what?? Then BOOM BOOM "we're taking damage sir!" "we have heavy flooding sir!" "Medic!" spinning screen rapidly fading to black. Imagine real life... war is not good, and should not happen. Alas, man is greedy and wants what his neighbor possesses, so it shall be until we die.
"whoah!!! where did this conversation go???"... lol - sorry