The Speed of Sound is determined by the density of the material it travels through. The denser the material the faster sound travels through that material. Water is much more dense than air, so sound travels much faster in water. About 4 times faster in fact.
The above effect changes our ability to determine the origins of sounds underwater. In air, our brain figures out from which direction sounds come from by measuring the time delay of those sounds in reaching our ears. In water, there is much less of a delay, so our brains canít figure our where sounds are coming from. Above water, a blindfolded person can determine the direction of sound to 10 degrees. For scuba divers, this is impossible.
Underwater, yelling does not transmit sound very well. This is because the sound vibrations do not transfer from air to water very well. Instead, to get someoneís attention, divers will rap on their tanks. When metal vibrates, the vibrations are transferred directly to the water and can be clearly heard, even if its direction cannot be determined.
All images courtesy Corbis Stock Photography. Created in 2003 by David Baxter & Timothy Schmidt.