No, you breathe the same as normal. You just get more molecules of air in your lungs each time you take a breath. In theory, you'd think you could breathe more slowly since you're getting more oxygen per breath. But when you hold your breath, and your chest starts the hurt, that pain is actually caused by the build-up of CO2, not from the lack of oxygen. And CO2 builds up at the same rate when you're diving, so you keep breathing as normal.

Also, an interesting fact: the deeper you go, the less time your air tank lasts. So if your tank would last you 40 minutes right at the surface, down at 100 feet it would only last 10 minutes.