You can experience overexertion when swimming against a current for long distances or carrying excessive weight. You may feel tired, short of breath, and weak. You have a tendency to cramp and feel panicky. What do you do?
Stop, breathe, and think about the situation.
Work out a rational approach and implement it. For example, overexertion at the surface requires you to establish buoyancy, rest, catch your breath, signal for help, and/or move slowly to the boat or shore. If overexertion occurs underwater, stop, breathe, and think.