Like, if you're underwater, you can't hear someone talking above the surface nearly as well. What causes this?

Answer 1

Geddy_V's answer is part of the picture. Water is much denser than air (by about a factor of 1000), and the speed of sound is much higher there. When a sound is coming from outside the water, most of the energy of the sound wave is reflected off the surface of the water back into the air, and so never makes it below the surface... that is why you hear very little of it. The small amount that does make it through is refracted and moves very quickly through the water.

The opposite effect happens if you are underwater and a sound also originates underwater. In that case, the sound propagates very quickly and efficiently through the water: you can hear underwater sounds VERY well when you are underwater too.

Answer 2

Sounds travels at different speeds in different mediums, so changing the medium from air to water will change the speed of the sound.