Here are a few things to think about that may explain why some submerges cause no damage and others do:
1. The temperature difference between the iPhone and the water.
2. The voltage between conducting materials that are bridged by water.
3. The conducance of the water. Pure water has high resitance, saltwater has far less resistance.
If the iPhone has only small entryways for water, and air inside, then contact with hot water will expand that air inside the iPhone creating more pressure than the pressure of shallow submerges and it will bubble out into the water preventing water intrusion.
If the iPhone is cooled by water that is colder than the air inside the iPhone then the air inside will contract and the outside pressure will push water in and the inside lower pressure will suck the water in.
Did you notice the posts about dropping the iPhone in bath water (warm) and it still worked?
When I was working for Icom America as a service tech I demonstrated how 1 micro liter of water in the right place could eat the foil off the logic board (with 3 volt memory backup battery) of a marine radio in about 20 seconds. Voltage and conductivity result in electrolysis.
Speakers need airspace behind them to work efficiently. If the case is airtight, the differences in pressure (due to atmospheric and temperature changes) between the inside and outside will distort the speaker diaphragm distorting or stopping the audio. In this case the case MUST leak air at least to balance the pressures and water is not far behind.
Due to my experience with water resistant radios, I note with dismay the focus on the time/depth of submersion and the lack of any attention to temperature differences that result in pressure because thats what moves the water, particulary where entryways are so small gravitational forces become irrelevant compared to pressure differences across small entryways.
David