Those instance may balance out those occasions when you don't get credit for holing a ship because SH3 thinks it sank from simple flooding. I've encountered situations where I torpedoed the ship, watched it slip beneath the surface, got credit for the kill, but the red sunken ship icon is not visible on the chart. Go figure
The ones that cause me the most consternation are the "bobbing sinkers". I've only experienced this in multiplayer, but you've got a ship that's 90%+ underwater and it appears to about to roll over and slip under, only to keep bobbing back upright. It's like a Monty Python "it's only a flesh wound" ship. These things, if they are gun equipped, will still have their guns manned and will shoot you if you surface. You'd think each bob would wash the men off the mounts, but they must be lashed to their stations with scuba gear.

Eventually you can put enough torpedoes in them to effect their sinking, but it will take far more than it should. I haven't experienced this offline.