In synchronized swimming, one plays closing one's nostrils pinching the nose. This is not simply for preventing water coming in from the nostrils, rather indicates that one should not inhale the water through the nose.
One must not breath through the nose when swimming. The modern medicine explains that the water through the nose compress the eustachian tubes, and it makes one loose the sense of balance and drown as a result. Water has the much thicker density of qi than air, so it is impossible to inhale such qi through the nose in one breath. That's why even if there is no influence on the eustachian tubes, one feels discomfort, such as the pain in the nose. When one requires the supply of gas is rapidly, the body uses the mouth.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.