Hearing under water
The reason for hearing things differently under water is the higher density of water compared to air. This changes how fast sound moves, which has a noticeable influence on the human ear.
Basic knowledge for Open Water Diver* (OWD*)
Under water, the speed of sound is about 4.5 times higher than on land. Therefore you can hear sounds from much further away. Also directional hearing, i. e. location the source of a sound is no longer possible.
Knowledge for Experienced Diver** (ED**)
Sound waves traveling through air are totally reflected from the head due to the large density difference between the head and the air. We will observe the same effect when sound waves traveling through air and hitting the water surface. For example, during a thunderstorm we will not hear the thunder under water, because the sound waves are reflected at the water surface. Above water, the sound waves are therefore absorbed only through the eardrum. Using the difference in travel time of the sound between the two ears, the brain calculates the direction of the sound source.
Underwater there is nearly no reflection at the head, since the density of the head is about the same as the density of water. The sound is received over the skull bone and both inner ears simultaneously, making a directional hearing kind of impossible. This is particularly important in waters with boat traffic, as the direction and distance to the boat can not be determined by hearing.
The speed of sound in the air is about 333 m/s. In the water, however, it is about 1485 m/s and is also a little bit dependent on temperature and salinity.