Sound travels in other ways too. Sound bounces off hard surfaces. It echoes from the walls of underwater canyons. Just as a mirror reflects light, hard surfaces such as canyon walls reflect sound.
The amount of sound that is reflected and transmitted depends on the acoustic properties of the two media and on the angle of incidence. The more similar the acoustic properties of the two media, the less reflection and the more transmission occurs. The less similar the two media, the more reflection and the less transmission occurs. A seafloor of hard bedrock is a better reflector than watery sand. Therefore, sound waves will bounce better off the hard bottom.