SOUND UNDERWATER
If you were to close your eyes and I moved to a different spot in the room and made a noise, you would be able to locate my position without looking. How is that done?
The part of your ear that sticks out from your head (the Auricle or Pinna) acts as a locater. When a sound is produced the listener will turn their head until the sound is at its loudest. The loudness is at its peak when the pinnas are both facing the sound, capturing it and reflecting it to the small opening into the ear.
Sound travels about 770 miles per hour in air depending on several factors. The human brain is fast enough to be able to detect the difference in arrival times of a sound hitting one ear and then the other. When the arrival time is the same the observer is facing directly toward or away from the source of the sound. That is another way a person can tell where sound is coming from.
When a diver is underwater the pinnas do not function as they do in air. Since they are almost the same density as the water it's as if they were surgically removed from the head. The ears may also be covered by a hood. And, since the speed of sound in water is about 4 times faster (it is about 3400 miles per hour) the brain is no longer able to tell the direction of sound underwater.
Sound does travel further underwater as well. Banging 2 rocks together can be heard for miles underwater. Contrast that to sound in the air! So, you will be able to clearly hear your Divemaster banging their tank to get your attention, but it will not be evident as to he or she is located.