Why do sound waves travel faster in water than in air?

Basically yeah, it has to do with density but it's not as simple as "denser = faster". What happens is that sound travels by transmitting vibrations from molecule to molecule, and in water the molecules are way closer together than in air. When molecules are packed tight, they can transfer energy between each other much more easily and quickly. In air they're very spread out, so the wave propagates more slowly because it has to "jump" bigger distances between molecules.

Beyond density, the elasticity of the material is crucial. Water is less compressible than air (like, it's more "rigid" in a sense), and that means when a molecule moves, it transmits that movement very efficiently to its neighbors. It's like the difference between pushing someone standing in a packed crowd versus in a line with lots of space between people. In the crowd, the movement spreads faster because everyone's in contact.

I really learned this when I was camping years ago and went into a river with my coworkers to cool off. We realized we could hear each other perfectly underwater (with our heads submerged) but the sound sounded weird and distorted. That's because sound travels at like 1,480 meters per second in fresh water versus only 340 meters per second in air at normal temperature. The speed is almost 4 times faster. If your teacher didn't explain it that way, they probably focused on the molecules and density part, which is correct but incomplete without talking about the material's elasticity.