Why does sound not travel between media?

So imagine you're at the beach; you go into the water and the moment you enter the water you stop hearing anything from the outside world. The same happens vice-versa: your friend shouts at you from inside the water but you only hear the bubbles rising to the top.

So the questions are:

1. Why does that happen?

2. Would it also happen if you used a hydrophone?

• It's not true. There's reflection and attenuation, but sound most certainly travels across boundaries. Dec 11, 2015 at 13:22
• If you cannot hear a friend or child shouting on the beach, or a helicopter flying overhead, while your head is underwater near the shore, perhaps you might make an appointment to have your hearing checked. Certainly putting one's head underwater makes all abovewater sounds much quieter, but not inaudible. Dec 11, 2015 at 15:03

• Liquids are roughly 1000 times denser than atmosphere. With light, you get large reflections when $n$ is only a few multiples of 1 - imagine the reflectivity of an $n = 1000$ medium... Dec 11, 2015 at 15:50