It really depends whether you’re thinking sprint type events or long distance. Anyone can “overdraw” their energy in the short term using anaerobic respiration – very high output for a short time, your body will produce an excess of lactic acid and the process is not as efficient as normal (aerobic) respiration. Highly trained athletes can do this for much longer periods.
For long distance swimming, the swimmer is breathing hard but is, in that sense, breathing normally. Again with athletic endurance training, the body becomes more efficient at many things. In fact many athletes train at high altitudes just to get their bodies used to dealing with lower oxygen levels.
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