CJ, your post triggered a thought that may be slightly off topic. I'm putting it out there so that somebody who is talented and in a position to riff on the idea and make it practical might see it.
It is hard to tell one object from another with sonar. It's not impossible. I wonder if it would be possible to build two permanently affixed nodes on the recorder packages that resonate at specific frequencies in a relationship of perhaps 1.7 to 1 in frequency.
I suspect that ratio would hold with pressure. And it would give you, in effect, a nice passive transmitter as part of the boxes. A chirp radar should be able to pick then out of most any other sonar returns that might happen.
If this is a good idea, I hope somebody takes it and runs. I'll shut up now.
{^_^}