I did a little more digging into underwater ultrasonic digital communication. There is a publication from DTIC which looks interesting. It includes some rather boring source code, but provides a lot of good advice in the text.
Available for purchase is An Ultrasonic Sensor Based Low-Power Acoustic Modem for Underwater Communication in Underwater Wireless Sensor Networks by Heungwoo Nam and Sunshin An. It's €25 for the publication, so I'll probably purchase it (unless some anonymous person with access would email me.)
Of course, this is leading to the ultimate conclusion of setting up an EE workbench and buying a bunch of expensive equipment (solder station, oscilloscope, function generator, multimeter, etc.)
I seem to have expensive hobbies. :-( I should really spend some money on a good vacation, which I sorely need.