Sound travels faster underwater.
In seawater, sound moves more than four times as fast as it does in air.
A sound wave moves through seawater at about 1,500 metres per second. In air it travels at around 340 metres per second. The exact speed depends on conditions in each medium.
In the ocean, changes in temperature and pressure can bend sound waves. In some parts of the ocean they form a natural channel, letting low-frequency sounds travel very long distances.