The loss of your RC submarine is always on the mind of the RC sub skipper. 
 
One manner of failsafe is the implementation of a high-output buzzer that will aid in recovery in the loss of the boat. These piezo speakers are exceptionally loud with 95DB output continuous alarm. They operate on anything from 3 to 20VDC. 

You can either wire this in to main battery voltage or have it operate on it's own battery source. If you mount to your watertight compartment exterior surface, sound transference should be exceptional. You can also mount this in the wet area of your submarine, as the design should be waterproof. You can set this up to continuously operate during your RC submarine mission, or wire it to come on in the case of loss of signal (with compatible RC radio gear). 
 
In practical application, in the loss of your boat, anyone can enter the water and listen for the siren. Unfortunately, the speed of sound through water is too fast for our human ears to triangulate relative position, but we can determine relative volume. Simply swim in a search pattern, listening for the siren. If it gets louder, you're getting closer. If softer, you're moving away from it. 

Cheap and easily implemented insurance for your expensive submarine!