I was looking at the tether discussion in the blog area and noticed that the fail safe issue came up. Personally, I would be pretty disappointed if I lost an ROV worth several hundred dollars at the bottom of a lake or something. I'd like to know what sort of ideas other people have at this point.
I was thinking a small high pressure air tank or CO2 tank could be used (paintball gun tank?). When a communication failure was detected, a valve could open and release most of the air in the tank. The loss of weight of the air while the tank maintained it's volume might create enough buoyancy to lift the ROV. The expelled air could also be captured and used for lift, but that might make a more complicated system.
What other solutions are out there?