Are they looking for a 37 kHz radio signal or audio signal?
Radio is an electromagnetic vibration and is highly attenuated by water. Sound waves are acoustic and propagate very well under water. The DFDR and CVR are equipped with an acoustic device radiating a signal in the ultrasonic region of the audio spectrum.
An oscilloscope connected to a hydrophone through an amplified bandpass filter would be able to detect and display the pings of the ULB.