In order to stay underwater we need a means of breathing in order to supply our body with the necessary oxygen. What makes this task difficult is pressure.
Pressure is defined as the amount of force per unit area.
This means that the air surrounding us is pushing on each square inch of our body with 14.7 pounds due to its weight. These 14.7 psi is what is called one atmosphere. The same thing happens in the water. Since water is much denser than air it takes a lot less sea water to excert enough pressure to total 14.7 psi. This means, that if we dive to a depth of 33 feet our body is exposed to 2 atmospheres of pressure.