All vertebrates have inner ears with similar components, but aquatic vertebrates have modified some of these to hear well underwater. Scientists are trying to understand exactly how ears of each aquatic group work.
Sound waves are characterized by compression and expansion of the medium as sound energy moves through it. This represents the pressure component of sound. At the same time, there is also back and forth motion of the particles making up the medium (particle motion). Animals can detect these different aspects of sound. Scientists are trying to unravel exactly which of these aspects of sound different species can detect.
Particle motion is described by displacement, velocity, and acceleration. All fishes detect particle motion, while some also detect pressure. (For more on fish hearing see: How do fish hear sounds?)
Most fish tissues have a density that is approximately the same as water and move with the impinging sound wave.
The otoliths (literally, “ear stones”) in the inner ears of fishes are significantly denser than other fish tissues. Because of their higher density, otoliths move with a different amplitude and phase than the rest of the body. It is this relative motion between the otoliths and the fish’s body that bends stereociliary bundles of the hair cells and allows fish to detect sounds.
Some invertebrates, including cephalopods (octopus and squid) and decapod crustaceans (lobster, shrimp, and crab) sense the particle motion component of sound.    It has been suggested that this is done using an organ called the statocyst, that primarily detects gravity and motion of the animal.
It is known that different species of fish vary in their relative use of pressure and particle motion to detect sound. Some fishes that have a swim bladder mechanically coupled to the inner ear are primarily pressure detectors, whereas fishes without a swim bladder only detect particle motion. These are two extremes of a continuum between only detecting pressure, such as goldfish and catfish, and only detecting particle motion, such as flatfish. Most fishes, such as Atlantic cod and salmon, fit somewhere on that continuum.
Some research suggests that pinnipeds and cetaceans are sensitive only to sound pressure in water. It is difficult to be certain if this is true, because all land mammals that have been tested use both particle motion and pressure, and it is difficult to do similar experiments with aquatic mammals.   
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