The difference is this: in a community centre pool, a diver doesn't give you compressed air to breath at the bottom of the pool. The thing the commentor above

The difference is this: in a community centre pool, a diver doesn't give you compressed air to breath at the bottom of the pool. The thing the commentor above is concerned about is pulmonary barotrauma, a.k.a. "burst lung". Between 10 feet depth and the surface the gas in your lungs will expand by 30%. If you took a deep breath at 10 feet deep, and held it, then that expansion can cause rupture of the alveoli. For this reason one of the first things you are told (or should be told!) in a scuba diving course is to breath continuously, i.e. never hold your breath.