(some) Submarines have one or more emergency beacons which automatically detach if they reach crush depth, rising to the surface and starting to transmit. In a

(some) Submarines have one or more emergency beacons which automatically detach if they reach crush depth, rising to the surface and starting to transmit. In a very shallow area the sub might be sitting on the bed but not below crush depth, so the buoy would not automatically detach. Of course it can also be operated by the crew, but it's really an absolute last resort thing.