When going scuba diving, physics becomes extremely important because the point is to be swimming with the fish! The goal is to be completely submerged in the water, thus in a state where the volume of the fluid displaced equals the volume of the object. Buoyancy force becomes extremely important because you want to ensure that the force you exert downward is greater than, and then equal, to the upward force exerted by the water.

When on the surface of the water, the scuba diver wants to increase his/her weight to be greater than the buoyancy force so that they sink. Later on, once the diver finds a comfortable point in the water they want to have the ability to decrease their force downward so that they can equal the buoyancy force and maintain their height in the water. 

To reach this goal, divers use a buoyancy compensation device (BCD) which gives the diver more control within this system.

The newer devices are quite complex, however, the concept is still the same. The device works by inflating and deflating (with air) to change the way that the forces interact. When adding air to the BCD, the more buoyant you will be, and the more that you will float. However, when you decrease the air in the device the more that you will sink. Overall, the diver can use this system to change the way that the buoyancy force and the weight of the person interact, thus changing how the body behaves in the water.