Underwater gliders seem to be promising. They can be used by submarines as passive detectors of surface threats such as the ACTUV mentioned above with passive sonars. When actuv is detected the small underwater glider drone can go to proximity of surface drone and detonate itself. An active protection measure against detection perhaps.
If the drone is close enough it can communicate data with the mothership submarine to a degree. thus controllable under water.
If the drone is close enough it can communicate data with the mothership submarine to a degree. thus controllable under water.