How do animals hear, especially underwater?
To understand how animals can hear underwater first remember how sound moves - like the energy through the pendulum balls.
Sound is acoustical energy transferred through matter. This transfer can be felt as variations in pressure or as the motion of particles. Air is compressible and less dense than water so animals on land sense sound more through variations in air pressure rather than in the motion of the air.
Water is not compressible, and is much denser than air, so sound in water is sensed as both pressure variation and particle movement. Also, because water doesn't compress, sound moves very efficiently. Sound energy does not dissipate as quickly in water, so it can be heard easily over long distances.
In air or in water, animals have evolved to hear in their particular environments.
Particle motion
Particles oscillate in place as they transfer acoustic energy and the oscillation moves the energy to nearby particles in a wave.
Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State
Pressure Variation
Changes in air pressure move an ear drum, which converts them to mechanical movement that can then be sensed in the inner ear.
Animation courtesy of http://www.edu.pe.ca/gray/class_pages/krcutcliffe/