A person can breathe underwater by using a snorkel or a scuba tank and regulator. Some people train themselves to hold their breath for long periods of time for freediving. Those who freedive use no external breathing support, but stay underwater for long periods of time.
A snorkel is a simple tube with a mouthpiece. The snorkeler fits the mouthpiece inside his mouth and lets the other end of the tube stick out over the surface of the water. When using this, the user has to stay right at the surface of the water in order to continue breathing. Although the user can dive under the water, he is unable to breathe until he surfaces and empties the tube of water.
Scuba diving allows a person to stay underwater much longer on a single tank of oxygen. Through a series of tubes and the parts of a regulator, divers can breathe underwater for around an hour at a time. A regulator has multiple stages and parts. The second stage is the actual mouthpiece of the equipment. This piece ensures the air is at a breathable pressure and features a back-up section where air can be shared with another diver when necessary.