Shock waves travel in a spherical form in all directions until reflected back by any objects in its path and, in this case, the mud line, the water's surface and delineated water density changes at a thermoclines. This relatively shallow water, well mixed by wind and currents may not have a pronounced thermocline. Hence the bulk of the wave power radiates away from the source in a circular pattern, most of which remains in the water. (I had to say something. My father was a sonarman in WWII Pacific Fleet)
Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."