Swimming uses your entire body and requires a significant amount of exertion. A runner or cyclist could tell you that those activities feel much different depending on whether the air temperature is warm or cold. The same holds true for swimming. The temperature of the water affects your performance, including your speed, but the time boost may not be worth the cost to your body.
Cool Water and Your Body
Terry Laughlin, founder of the Total Immersion method of swim instruction, explained in an interview how the body responds to cold water. Initially, blood vessels dilate, allowing warm blood to heat the extremities. Then, in order to preserve the core temperature, the vessels begin closing to keep internal organs from getting chilled. However, after a while, vessels will open again because they cannot maintain the constriction. Cool blood flows from the extremities back to the core, and at that point, you have to take precautions, like getting out of the water, to avoid hypothermia.
Warm Water and Your Body
"A hot pool does not remove the excess heat (sweat) a swimmer generates and therefore they expend more energy exponentially," Roger Bacci, a commercial aquatics expert, writes in his article "Competitive Pools." This can cause unhealthy levels of exertion, and your body can overheat. Since open water swimming tends to involve longer distances and duration than indoor events, without rest breaks between laps, Laughlin recommends swimming in a cooler temperature than you would expect in a pool. In his experience, "You can acclimatize to colder temperatures, but not to warmer ones."
Standards for Swim Competitions
The Federation Internationale de Natation, FINA, is the governing body for competitive swimming throughout the world. Water temperatures for indoor events must be between 25 and 28 degrees Celsius, or between 77 and 82.4 degrees Fahrenheit, respectively. The optimal temperature is 26 degrees Celsius or 78 degrees Fahrenheit. In December 2010, The New South Wales Technical Open Water Swimming Committee in Australia adopted a policy for open-water swimming to cancel events if the water temperature is higher than 29 degrees Celsius, 84 degrees Fahrenheit.
Studies and Conclusions
The "Journal of Sports Medicine and Fitness" published a study in 1993 that showed that warmer water did in fact increase speed. However, author V. Mougios noted, "the augmentation of performance efforts in the warmest water is accompanied by greater metabolic and cardiovascular loads." In other words, warmer water makes your body work harder. While the study showed a temperature of 32 degrees Celsius, 89.6 Fahrenheit, yielded faster speeds, experts seem to agree that a cooler water temperature is healthier.