"The secret to underwater swimming is going deep early. The trainees learn that if they swim along the bottom in deeper water, the increased partial pressure of

"The secret to underwater swimming is going deep early. The trainees learn that if they swim along the bottom in deeper water, the increased partial pressure of oxygen in their lungs will allow them to hold their breath longer and swim farther." That, plus the somewhat more streamlined profile of the upper body when under pressure. For example, blow up a balloon at the surface and bring it down to the bottom of a swimming pool and notice how it shrinks. The same thing, although much less spectacular, happens with your chest. (This is not the case when you use scuba breathing gear, which will allow you to keep the pressure of the air in your lungs equal to the pressure of the surrounding water at the depth you're at).