You need a Sub-Saver device. It is a relay that at a no-signal condition will either blow a ballast tank or set the planes to full-up/ full speed depending on how it is installed.
Most R/C subs rely on internally generated heat to maintain positive pressure inside the hull, but a clever builder (if space is available) will install a shrader-valve and pressurize the hull a bit before putting the sub in the water. I can get a reliable 6' of depth under most conditions. By that, I mean have full controls and all functions.
Remember the principle of the Cartesian Diver... that is where many sub operators run into trouble with negative bouyancy. Remember, sinking a submarine is well within design parameters, and almost anyone can do it - the trick is "unsinking" it afterwards.
Peter Wieck Wyncote, PA