Watch a bow-shot ship sinking by the bow, and the screws are still going as they go under. They're "driving" themselves under. I've followed several down until I couldn't see them, and the screws were still going, even after massive explosions. Then there's the sinking noise which follows them down all the way. So if they're in deep water, and you're still within render range, it may well be using system resources on the computer and creating quite the racket... I see a CapnScurvy style "Ship Sunk Deep-water Sonar Test" and a "Ship Sunk Shallow-water Sonar Test" set of missions coming up...
- Don't use the AI subs though Capn - they don't make enough noise underwater...