As it currently stands, depth charges do direct damage to a submarine, filling in for torpedo slots.  The damage varies by sub type, but typically two or three is enough to sink most submarines out there. Four might be the max for the top tier subs.

However, this really isn't how depth charges work. Fact is, rarely did a depth charge do direct damage to the submarine they were targeted on.  Rather, the explosion of the charge (200 pounds of explosives) created a shock wave in the water which would cause flooding (due to pressure) in the submarine.  This would cause the submarine to go from neutral bouyancy (submerged) to a state of negative bouyancy (sinking). Unless the crew worked fast to stem the leaks, submarines would drop below their crush depth and implode like a crushed tin can.

I propose the following mechanics change to depth charges based on the above facts.

When a submarine is struck by a depth charge, flooding happens. This forces the submarine into a critical dive, and prevents surfacing.  The player has a set time frame to either a.) fix the flooding with crew, or b.) attempt to critical surface (or blow as submariners call it) to quickly surface.

If however the player is unable to stop the flooding, the submarine remains at critical depth and takes damage accordingly, ultimately sinking.

Once the blow has been performed, due to expending all of its ballast tanks, the submarine is treated as having surfaced with zero air supply, and must deal with the situation accordingly