A human being floats on water, because the specific gravity is around the same or rather lighter than water. One gets lighter in the sea water furthermore, but it is notable that there are people who are called "hammers" (note: those who cannot swim are called "hammers" in Japanese ).
This is because lacking experience in water makes one's consciousness upset, and the body gets harder more than necessary, and as a result it makes impossible for him/her to adapt to water. This can be understood as one's consciousness thickened density of qi, so that the body gets heavier than water.
It can be said that the flexible movement of qi lightens the body, makes the movement of the body smooth, lessens the stress to the body, this is largely related to the state of consciousness.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.