I've found foundering ships several times in one of my last campaigns. I always found the Aquitaine in convoy struggling, almost sunk. If I waited long enough I could find her alone behind the convoy. Can't wait too long though or she'd sink on her own and I'd get no credit!
I actually downloaded a mod that supposedly adds fire damage to burning things. That is, as they burn, they take more and more damage. But I'm skeptical as to whether it is actually doing anything or not. Just the other night I watched a burning B-17 fly on until it was gone, and I've seen many burning ships carry on just fine also.
Steve