We don't yet know why US swimmer Fran Crippen died in Dubai, but the most likely cause is heatstroke. He disappeared in 86 degree water barely 400 meters from the finish of the six-mile World Cup race.
You are far more likely to suffer heart stroke racing in water temperatures above 80 degrees than you are to suffer heat stroke at any temperature on land. During exercise, more than 70 percent of the energy used to drive your muscles is lost as heat, so your heart has to pump extra blood from your hot muscles to your skin where you sweat. When you exercise on land, sweat evaporates and cools your skin to dissipate the heat. You produce sweat when you swim, but the sweat cannot evaporate to cool your body.
Almost all cases of heat stroke occur when you suddenly increase the intensity of your exercise, like the finishing sprint of a race. Nobody should ever die of heat stroke because your body sends you warning signals as your temperature rises. In 1965, I almost died from heat stroke in an unimportant local running race in Arlington, Virginia. I am still embarrassed by my stupidity because I ignored all the warning signs as my temperature continued to climb. First your muscles are affected, then your circulation and then your brain. As your temperature starts to rise, your muscles feel like a hot poker is pressing against them. It is normal for intense exercise to make your muscles burn, but hard exercise does not cause painful burning that feels like fire. Furthermore, the burning of hard exercise is relieved by slowing down. The muscle burning of impending heat stroke does not go away when you slow down.
As your temperature rises further, the air that you breathe feels like it's coming from a furnace and no matter how rapidly and deeply you try to breathe, you can't take in enough air. When you exercise intensely, you can become very short of breath, but the air you breathe will not burn your lungs. Burning in your lungs, not relieved by slowing down, signals impending heat stroke. When you feel that the air is so hot that it burns your lungs, stop exercising. Your heart cannot pump enough blood from your exercising muscles to your skin so heat is accumulating rapidly and your temperature is rising rapidly. Your temperature is now over 104 and continuing to exercise will raise your body temperature even further and it will start to cook your brain. Your head will start to hurt, you'll hear a ringing in your ears, you may feel dizzy, you may have difficulty seeing and then you will end up unconscious. Your temperature is now over 106 and your brain is being cooked like an egg in a frying pan.
When a person passes out, call for medical help immediately. If it is heatstroke, he should be cooled immediately, but if it is a heart attack, cooling can be fatal. Carry the heatstroke victim into the shade and place him on his back with his head down and feet up so blood can circulate to his brain. Cool him by pouring on any liquids you can find or spray him with a hose. As you cool him, he will then wake up and talk to you and act like nothing has happened. While he's sitting or lying there, his temperature can rise again and he can go into convulsions or pass out again, so he must be watched for some time afterward.