The decision on how warm to keep the pool is up to the individual owner. The pool temperature recommended by the American Red Cross for competitive swimming is 78° F. However, this may be too cool for young children and the elderly who may require 80° F or higher. The typical range is 78°- 82° F.
(Keep in mind that the energy consumption for each degree rise in warmth will cost from 10 to 30% more in energy costs, depending on your location. In warmer climates the percentage is higher due to the relatively low cost of heating to 78° F.
If you are vigorously training, your body generates an increased amount of heat energy. In air, this heat is dissipated by sweating. The evaporation of the sweat is the cooling mechanism to help control the body temperature at 98.6° F. In water, your body generates the same amount of heat, but there is no evaporation to provide cooling. Cooling is provided by conduction/convection of heat from your warm body to the cool water. This is roughly proportional to the temperature difference between your body and the water. The warmer the water the less heat is dissipated.
This is a difficult question to answer because it varies based on individual swimmers preferences. Based on recommended heater temperatures and settings from virtually all manufactures, the ideal range for both indoor and outdoor pools is anywhere from 75 degrees to 85 degrees Fahrenheit. This figure should not be thought of as final as temperature and comfort does differ from person to person and environment to environment, but it a good goal to reach.
If you have a backyard swimming pool without a heater, looking for an ideal temperature is a difficult task. The temperature will gradually increase as the season changes and as the pool heats up, but the initial temperature in the early parts of the swimming season will be quite cold - usually well less than 70 degrees Fahrenheit. Even though there is no proper pool temperature, water temperature less than 65 degrees Fahrenheit are unpleasant even for the most adventurous swimmers. On the other side, if you do have an outdoor pool with a heater, set it at something that is comfortable to your swimmers and your activity level. For instance, cooler water is more ideal for exercising where warmer water is better for a lounge / beach atmosphere. But, keep in mind that turning up your heater too much can lead to unexpected drama.
When you increase the temperature of your water, it can have an effect on pool maintenance. With higher temperature comes more water evaporation. This means you will need to be more diligent about filling up your pool to keep the water in your skimmers. Higher water temperatures also mean a faster buildup of dissolved solids. This is basically the "bad stuff" that the chlorine attacks in water. Higher pool water temperatures will also burn off your chlorine and pool sanitizing agents much more quickly. Algae also love the hot water so they will grow faster and force more serious and diligent maintenance such as vacuuming, squeezing, and cleanup.
Even if you don't have a heater to maintain your temperature, the same concerns of a warmer pool will eventually plague your pool in peak summer heat environments. Keep an eye out for seasonal changes and how they will affect your water chemistry. The most important chemical water reading in during hot and cold times will be your total alkalinity. When the weather is cooler, your total alkalinity should be increased. In warmer weather, your total alkalinity should be decreased with a muriatic acid to keep the saturation index fairly constant.
There is no ideal swimming pool water temperature but you should adjust your temperature based on your individual preference. Also, keep in mind the added responsibilities that come with maintaining a warmer pool. Watch out for seasonal changes and note how they affect your water chemistry. Both cold and warm temperatures have an effect on pool chemistry and maintenance routines.