Well the obvious answer is that bodies naturally float. When you inhale, the body expands and makes it more likely to float. This means that an efforrt has to be made to swim underwater.

An experienced diver will tell you that the best way to stay submerged without aid is to breath in a fashion that makes blood oxygen rich, exhale and then dive.

This is also the reason why divers use weights to remain submerged. It is a simple trick because if you do not do this you will also have to make an effort to stay submerged.