But against subs I'm surprised there aren't plans for a drone-net that surrounds a fleet or high value ship that can detect incoming underwater threats via some mesh networking.
That's also a major issue for a lot of other drone swarm schemes such as using drone swarms to counter fighter aircraft. Smaller drones don't have the propulsion to keep up with fighters, and once you make drones large enough to hold the engines that would let them keep up with a fighter you might as well build a fighter instead.
I wonder if you could have a flotilla of heli-drones that drop in sampling microphones/active sonar. Wind might be hard to deal with in storms or heck even typical conditions.
It is interesting to consider the change in tactics and design once you consider the fighter expendable. You don't have to turn away once you get shot at in-range -- you can keep flying and shoot at the point of maximum impact. The aircraft might only be designed to last for 10 combat hours, or to have a poor safety record. For example, in aircraft the design margin is ~1.5 (vs >2 for surface); for unmanned it might be ~1.1.
The drone-vs-human tactics will persist until both sides have competent drones, at which point they might evolve to look more like human-vs-human tactics: protect your high value assets, imperil theirs.
Fundamentally underwater hydrodynamics is still in its infancy. How do I know? I just have to look at a shark.