Assuming this is unchanged from SH4 (I did a LOT of work on SH4 DCs), detonate_depth does nothing for ship-launched DCs (except HHs).
Air-dropped DCs use this functionality.
The AI overrides this value, and sets the charge where it thinks you are. This was important for the PTO because IJN DCs only had 2 depth settings until 1943, when they got a 3d—all of which were too shallow.
In addition, the depth precision is the range at which they explode, but really they need a fuse precision as well. If the DC could only be set in 5 meter "ticks" then they'd have to decide to drop some at one depth, some at another. Then some randomness on top of that.
Also, you need to be careful about the radius because if the sub is shallow, there is no MINIMUM setting for depth, so you're at PD, and the escorts drop DCs that instantly go off, and wreck the escort (sinking it sometimes, but often blowing up the DC launchers).
DC thread
http://www.subsim.com/radioroom/showthread.php?t=123307