The emergency ascent system would monitor things internally on the ROV such as temperature, humidity, and maybe pressure. This way if something went horribly wrong on the ROV, it would try to surface immediately.

It could be something as simple as a 555 timer tied into the signal lines running to the altitude motors. This would assume that the motors run off of a servo type signal though. The 555 timer circuit would be controlled mostly by the emergency ascent related sensors, and the microcontroller on board. I.E. just the humidity sensor could trigger the emergency systems if the microcontroller became locked up.

Is there a way to have the watchdog trigger a pin if the ROV's built-in microcontroller becomes locked up?