Sound Travels Four Times Faster in Water Than in Air
Sound is a wave of alternating compression and expansion, so the speed of it depends on how fast it bounces back from each compression; the less compressible the medium it’s traveling through, the faster it bounces back. Water is about 800 times more dense than air, so there are way more particles for waves to bounce off. Thus, sound is faster in water.
However, the density has the opposite effect on physical bodies (such as, say, a bullet). Physical matter encounters drag when in the water due to its density, as laid out by the drag equation, in the seminal An Introduction to Fluid Dynamics.
It’s been proven that jumping into the water and swimming within three to eight feet of its surface will literally save you from catching a bullet (all those movies and crime shows you see people jumping into the harbor on the run have a scientific basis after all!).
