Pressure of air at sea level = one atmosphere or 760 mm hg. The weight of a column of water directly above a diver"s body (hydrostatic pressure) increases directly with increasing depth. The pressure increases by one atmosphere for each additional 33 ft. of depth. Because the tissues of the body are largely water, they are non-compressible. However, the body contains air cavities - lungs, respiratory passages, sinus and middle ear spaces - where volume and pressure will change with increases or decreases in diving depth. Boyle"s law - the volume of any gas varies inversely with the pressure on it (ie) if the pressure is doubled, volume is halved. There are limits to snorkel size because: pressure effects - when breathing through a snorkel, the diver must inspire air at atmospheric pressure. Dead space of regular snorkel = 150 ml. At rest: 350 ml = 500 ml - 150 ml.