I have designed ROVs (I was involved in the design of JASON, the robot that discovered the Titanic) and I have designed AUVs (Autonomous Underwater Vehicles.) Here is how we provide for emergency recovery. For Tethered vehicles like an ROV, you pull on the cable to recover it.
For AUV's with no cable attached, you make your vehicle neutrally bouyant or slightly negatively bouyant with a drop weight attached. The dropweight is held in place by a "burn wire". To release the dropweight you run current through the burn wire. It burns and releases the dropweight. The vehicle then floats to the surface.
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I have designed ROVs (I was involved in the design of JASON, the robot that discovered the Titanic) and I have designed AUVs (Autonomous Underwater Vehicles.) Here is how we provide for emergency recovery. For Tethered vehicles like an ROV, you pull on the cable to recover it. For AUV's with no cable attached, you make your vehicle neutrally bouyant or slightly negatively bouyant with a drop weight attached. The dropweight is held in place by a "burn wire". To release the dropweight you run current through the burn wire. It burns and releases the dropweight. The vehicle then floats to the surface.