Water is thicker than air, so sound moves much faster through water, approximately four times faster. The way that sound moves through ocean water is affected by the temperature, salinity (amount of salt) and depth (pressure). An increase in temperature, salinity and/or depth leads to an increase in the speed that the sound travels. But, these factors sometimes work a bit against each other. The surface is warmer than the deep ocean, but the pressure gets greater the deeper you go. So, figuring out the exact speed isn’t always easy.
One of the most interesting features of the ocean that relates to the movement of sound is a channel between 2600 – 3300 feet (800 – 1000 meters) that allows for sound waves, especially low frequency sound, to travel really long distances. But, it doesn’t travel as fast as at other depths. This is known as the SOFAR (Sound Fixing and Ranging) Channel. Baleen whales seem to be able to use this channel to communicate across ocean basins.