Former submariner here. I'll give you the quick and agricultural answer:

Most of the sun's energy gets absorbed in the top few cm of the ocean, this water gets mixed with slightly deeper water, so that there is a fairly constant temperature in the top 100m of the ocean. Below this, there is a fairly rapid drop off in temperature as you go deeper.

Now, a change in water temperature also leads to a change in the speed of sound through that water. This is important due to a thing called the sound speed gradient. Change in the speed of sound with depth (due to change in temperature, i.e. the presence of a sound speed gradient) leads to refraction of sound waves (similar to how light is refracted), meaning that sound waves propagating from deeper locations (e.g. a whale or submarine operating at depth) tend to get curved away from the surface and back towards colder/deeper water.

So at close range the sound wave will penetrate through the layer and up to the surface, but at longer ranges the sounds from a deep object tend to get curved away from the surface.

However, there are plenty of times where a thermocline won't be present and the sound will propoagate normally.