Just blue-skying here, as I've not once tried out--if available in my BAT 4.0 game--any depth charges...
If depth charges do have a delayed explosion after surface impact, and if an effect can have its point of origin set to below the water surface, then it should be possible to create a dedicated effect group for their detonation.
One part of the effect is set to occur at the surface. This could comprise the circular patch of disturbed/shocked water, and the initial, raised 'bulge'. The other part is set to originate some distance below the surface, it being a rising geyser that takes a little period of time to break through the surface. Any of you who've watched films of depth charge attacks will be well familiar with the sequence of events.
If the provisos mentioned are met, and if it's straightforward to differentiate a depth charge from other bombs and torpedos, then it should prove easy to implement such a new effect group.