Sound waves require a medium, but are not particular about the kind of medium. Typically we associate the pressure waves with air, as that is how we hear, but sound also travels through liquids and solids. In 2010 researchers at Caltech demonstrated a concentrated pulse of sound, a sound bullet, in a solid, but now they are back with a sound bullet in a liquid. Water specifically.
The lens the researchers developed for creating the sound bullets is similar to a Newton's cradle, which suspends identical metal balls in a line. When a ball at the end strikes the others, the ball at the other end is launched away. For the lens, striking one end creates compact nonlinear pulses of sound that travel as sound typical does until reaching the lens' focal point, where the separate waves coalesce into a single amplified signal. This amplified signal is the sound bullet and has a great deal of potential in multiple fields.
Sound waves are already common in biomedicine for sensors but a sound bullet could be used to target and destroy tumors, without damaging surrounding material. Also sound bullets could be used to find damage to ships, bridges, and other objects in the water.