Here’s an email sent to me the other day by Larry:
Even when I dive to the bottom of a swimming pool, I get pain in my ear. Is this common and how can it be prevented?
Hi Larry,
The pain you describe is caused because the pressure outside of your ears is different from the pressure inside the air spaces in your ear. If you do not equalize the inside and outside pressures you can do serious damage to your ears. Many people experience a similar discomfort when flying as a result of the differential between the air pressure shifts at various altitudes (usually takeoff and landings). For aircraft related discomfort it is usually recommended that you chew gum or yawn in order to “pop” your ears. Diving works the same way however I wouldn’t suggest chewing gum or yawning during a dive 
What yawning and chewing gum do is promote the opening up of the eustachian tube which allows the for the pressure equalization between the outside air pressure (or atmospheric pressure) and the internal pressure.
The eustachian tube is small opening that connects the pharynx and the middle ear. While going into a lot of physiology might be more than what you want to know, let’s just say that this tube or opening connects your middle ear with the outside pressure which equalizes the air inside of your middle ear with the pressure outside of your middle ear.
As I mentioned, you might want to avoid chewing gum or yawning while diving so the trick is to re-create the same effect which is opening up of the eustachian tube. Most divers do this by pinching their nose and gently attempting to blow out through the nostrils. Mind you, I said gently. It is not to be done forcefully otherwise you may do damage to your ears. A gentle blow will usually open the eustachian tube quite nicely. Other divers prefer to wiggle their jaws or to form an “O” with their tongue in their mouth which all induce the same response.
Sometimes if you have a cold or other physical response that causes either swelling or blockage it will be difficult or impossible to open up the eustachian tube to equalize the pressure. The only recourse is to abort the dive. I have yet to encounter any dive worthy of losing my hearing or doing other permanent damage to myself. The ears also control your equilibrium (balance) so you could end up stumbling around like a drunk the rest of your life if you damage your ears too.
All of the PADI entry level diving courses contain information air space equalization and it’s something you practice from your very first dive in the pool.
Bill