This is important... The speed of sound varies greatly depending on temperature, altitude, which media it's in, etc. It's doubtful that anything could approach the speed of sound in water.
I'd presume they refer to the speed of sound in air.
Still, faster than that while underwater sounds impossible giving water is some 800x denser than air at sealevel. I do recall some torpedo tech involving torps ejecting a thin film of air around the torpedo body as it moves with the effect being an enormous reduction of friction, enabling a torpedo to move some 200 MPH.
Next challenge after that is to make it still able to be self-guiding to a target.