Dolphis and Wales use sound waves... right?
What if you got some epoxy and sealed water tight a piezo speaker, have it generate different tones and depending on the tone, it performs different actions? Dolphis and Wales use sound waves... right?
Its dead easy and pretty difficult to go wrong.
I'd use a piezo buzzer because it would be louder. Yes/No? Then measure the sound waves in the pool and program submarine to change direction by hearing responses to sound waves frequencies. Communicate like a whale!