According to a lot of the historical books I've read (try your local library non fiction section, maybe you can learn everything) it was pretty common, not only to finish off damaged ships but to sink small ones that weren't worth a torpedo. Sometimes it backfired, I recently read an account of a gun attack on a sampan which turned out to have more and heavier weapons than the sub.
As for the torpedoes, I was in a heavy helicopter anti-submarine squadron, and we had 3 incidents on the same exercise of a Mark 46 torpedo running up the backside of a wave and flying into the air. I didn't personally see these, but the pilots claimed they flew up right through the rotor arc without hitting any of the blades.
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