May I ask a question  ??  Is there a mechanical interfaceto allow the underwater sound to break the surface ?Theearly phonographs used a needle (cactus spine) to move therecord's grooves vibrations to the diaphram in the 'head" andhence into the air.   I visualize  a megaphone with some passivedevice in the small end to couple the underwater vibrations intothe large end for radiation.  A "probe" would fit the use.Just as was mentioned earlier about the small bones in the ear.