Questions and answer about pings in water
Please be aware that the pings are acoustic (sonic waves) and not electromagnetic (radio waves). Water is not an ideal isolator, any electromagnetic emission is absorbed within very short distance.
Sonic (audible) pings have a carrier frequency in the low kilohertz band, ultrasonic waves may go up to 100 kHz. Modulation of such a "carrier" is physically possible but technically not feasible, since the "carrier" signal is far too weak to carry any useful and detectable modulation. In the deep sea, sonic waves do not propagate easily, they get deflected and echoed by changes in water temperature and density anomalies. But even if that problem would could be overcome, any modulated data transmission rate is intrinsically limited to a fraction of the carrier frequency, in an underwater application to something like a morse or telex transmission.
That's why the black boxes cannot transmit any data.