the speed of sound in water (and in air too, to a lesser degree) is intensely variable with factors such as salinity,depth, adjacent layers and such.
it is possible that you could be a few hundred feet away and miss sounds channeled away below. Some advantage could certainly be gained by varying the frequency (pitch) of the ping depending on depth,etc but would be very expensive with very little attributable value.
Best scenario I think would simply be saturating the earth and oceans along the flight path with relay stations (think cell towers) which could capture regular broadcasts (not satellite) of ACARS like information.
The cost of maintaining this network would be offset by having them serve some dual purpose (i.e. we already have ultra sound weather buoys to capture water temperature, weather data etc.)