Ummmmmmm, I think you'll find that the basic problem is that your torps were set too deep in the first place.
You stated the C3 Cargo has a draft of 12 meters, according to my game the C3 Cargo has a draft of 9 meters. So with your torps set for 10 meters you were already below her draft by 1 meter to start with. Let alone taking into consideration the sea conditions.
The only merchant that has a draft of 12 meters is the Passenger Liner with a draft of 12.6 meters, with the T3 Tanker next in line with a draft of 10.7 meters and the T2 Tanker with 9.5 meters. All the other merchants have drafts of 9 meters or less.
In general I've found the C3 Cargo goes down 75-80% of the time with my torp set to 6 meters depth and aimed just below the Bridge Deck (same spot the scope puts the triangle when you lock on). This is also the intersection of the Engine Room and Fuel Bunkers. If I'm shooting in Heavy Seas (winds of 15 meters per second) I'll set my torp depth for 5 meters and have never missed a C3 Cargo yet with my torp going under her.
I think if you double check the draft for the C3 Cargo you'll be hitting them a lot more often.
