If you use an ESC it probably turns itself off with radio "Loss Of Signal", as when you go too deep. I wouldn't feel comfortable in a deep pond, with a failsafe system that depends on a compressible bladder, which makes your sub heavier, and may not allow you to surface if your drive motor stops from LOS, or catches on weeds, or other hazard. It would be better to have the bladder flat, centered, and able to expand if your boat ends up "dead" on the bottom. This could be accomplished by a failsafe system that activates a CO2 cartridge or an air pump to inflate the bladder on LOS or at a predetermined depth.
RCABS- It's easy to control your own bladder!