What is the cause of charley horse on the foot when people swim?

Exertion, 'lytes. Charley horse is another term for muscle cramp. It has many possible causes, and can often be caused by intense or prolonged muscle activity, as could occur during swimming. Charley horses are more likely to occur when a person is dehydrated or deficient in certain electrolytes such as potassium, calcium, or magnesium.