When peeing into a toilet . . . why does a stream of urine sound more intense, when striking the part of the water with the greater depth? Wouldn’t the sound waves be coming from the surface, and therefore be the same in all locations? Or are sound waves produced, even below the surface? Or do the surface sound waves emanate downward as well as upward, causing a near-simultaneous echo when they reach the bottom?
You probably need to think about the shape of the bowl and the relationship with the point of impact with that shape. It is probably coincidence about the depth of water, and more to do with the effects of the bowl shape in focussing and matching the impedance of the sound production to the air.
Sound does propagate in water, but water has a vastly faster speed of sound than air, and is much denser. The impedance mismatch of the water to air interface means not a great deal of sound energy comes back out of the water into the air. The closer density of water to porcelain means more energy will tend to go into the bowl, and be dissipated into the rest of the structure.
The deeper the water the deeper the sound, or more accurately deeper bass tone. Drop a rock in shallow water compared to a rock in deep water. The deeper water resonates.
I use a catheter which empties into a bag so all of it sounds more intense. But that rule applies in all types of water.
Lukeinva is on the right track. In the deeper part of the toilet, the water responds to impact with more of its volume: find a guitar or other stringed instrument and compare plucking the string in the its middle as opposed to near its end.
When you disturb the shallow region of a body of water, the waves you create lose amplitude (height) as they cross deeper parts of the body (a really big tsunami may show only an inch or two displacement in the open ocean). By contrast, creating the same waves in deep water has the opposite effect, the waves increasing in amplitude in the shallower areas. I cannot quite express the idea coherently, but someone else probably can.