waves - 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?

   
   

Answer

1: It's caused by the change of propagation medium, more specifically it's characteristic impedance given by it's density and sound propagation speed. In this point of view the difference between air and water is huge. At the border between those two media there are strong reflection caused by the impedance discontinuty (that's why you can only hardly hear in the water from the air outside and vice versa - the power is mainly reflected).

2. Yes, it would. The difference between hydrophone and you ear is only the difference between sensors, not the principle itself.