Not sure what's up with the Hunt III destroyer, but it seems to be indestructable? I encountered it just after a save, so this actually happened to me twice, once originally, and the second time after reloading the save:
Perfect weather, ideal AoB, within 2,000 meters, with the torpedo set at ~4.2, based upon the draft of 3.7. Goes off just below the keel, and rocks the dessie like a toy (see below) . . . and nothing.
Dessie just keeps cruising along and, in fact, keep going in a straight line as if nothing at all had happened.