Draken;
A wild suggestion I guess but maybe you could use the Trigger objects to 'simulate' hitting a submarine.
For example;
Place "Message" triggers on the map and make the circumference small. If your aircraft flies through it on a depth-charge run you get a message stating "the submarine has been hit" or something similar. Combine it with a Trigger "Apparition" object and have a smoke object appear on the surface using the same method. It's too bad there isn't an "oil slick" object for the water that you could use (there's a less than subtle hint for the modders out there!!! nudge, nudge, wink, wink, saynomore!)
Along the same lines you could use the Trigger "Apparition" object and have a sub appear if you fly through the Trigger 'circle' simulating it being forced to the surface from your depth charge attack. The smaller you make the Trigger circle the harder it would be to do it if you get my meaning.
It would be a bit artificial but might be worth a try.