Unlike air, water cannot easily be compressed in nature. This is due to the molecular arrangement of liquids as compared to that of gases. Accordingly, water pressure is uniform across different depths, with only the weight of the water above that point dictating the pressure acting on any object under water.
Seventeenth-century Englishman Robert Boyle developed a fundamental law of physics, which says that "at constant temperature, the volume of a gas varies inversely with the pressure, while the density of a gas varies directly with pressure." Practical demonstrations of this law show that where a constant temperature is maintained, the volume of air in a container will halve when the pressure acting upon it doubles. This in turn doubles the pressure of the air itself. The pressure acting upon a container under water doubles at a constant rate according to the amount of water above the object, such that doubling the depth of an object will double the pressure on it and halve the volume of the air contained within it.
It is vital that scuba divers have an understanding of Boyle's law, as the pressure of air held in their tanks and in their lungs will be directly related to the depth of water at which they are swimming. Boyle's law explains, for example, why divers must not hold their breath as they return to the surface, as each time they halve the distance to the surface, the volume of their air within their lungs will double.