When I swim, I get warmer from the exertion. If swimming lowers your temperature, I assume it is because you swim in really cold water... What swimming will do is help prevent you from overheating as fast as you would in air on a warm or hot day. So, I suppose you could say that swimming helps keep your body temp from going higher because the water is more effective at conducting heat away from your skin than air - especially hot air. I do know that when I get into a pool that seems a little too cool, it starts feeling nice after a lap or two and not cool enough after serious exertion.

The way that the human body reacts to temperature changes is to protect the ability of our vital organs to function. A human's vital organs with the exception of the brain are located in what is commonly called the core which is the area from neck to crotch. Body temperatures of humans tend to be around 98.6 degrees fahrenheit. The body reacts to a rise in temperature by producing sweat. If the temperature of the core drops the body will react by shivering. The shivering is its attempt at making heat by friction. When you immerse yourself in water it conducts heat away from your body 20 to 25 times faster than in air all factors being equal ( i.e. 80 degree air temp and 80 degree water temp). The body will attempt to try to heat heat up the surrounding environment to a tolerable level. Without any thermal insulation the core temperature continues to drop and the body starts going through what is called vasoconstriction which is basically shutting off blood flow to the extremities (the arms and legs). Keeping the blood pooled in the core. the carotid artery which goes to the brain is not affected by vasoconstriction. Seventy five percent of the body's head loss is through the head. When the body begins shivering, it is a warning that you are losing body heat. When you stop shivering it means you are going into a condition called hypothermia. If the body is not rewarmed it will begin shutting down vital organs one by one with the brain being the last to shut down. Colder water heat loss happens faster than warmer water heat loss. Both, if left unattended will lead to hypothermia. I teach scuba diving just south of Chicago in the U.S. ( http://www.visibilityunlimited.com ) which is subject to a wider variety of temperatures depending on the season.

Unless you are swimming in water that is 98.6 degrees or higher, there is always the risk of a lowered body termperature. Even people whose boats sank in tropical waters have died from hypothermia once in the water long enough.