You have to choose the best, the safest, the most secured and the most suitable sport activity among the varieties...
That is...
Aerobics 增氧运动:

During swimming practice, the reduction of work of the metabolic process and the cardiovascular system cannot be done without a large amount of oxygen. Yet the oppression of water pressure over the chest and abdomen increases the difficulty of inhalation. Specialized test had been done and showed that, the thorax of a person has to bear 12-15 kg of water pressure while swimming is in the process. In order to allow the body to acquire adequate amount of oxygen, respiratory muscles must ceaselessly overcome this kind of pressure; furthermore, exhalation during swimming process is usually done underwater and the water density is much greater than the air density. Therefore, extra force must be applied in order to complete exhalation. Hence, contraction ability of the respiratory muscles can be increased through both inhalation and exhalation, which gradually enhances the function of the respiratory system and increases vital lung capacity. The vital lung capacity of a normal and healthy man is 3000-4000ml, while regular swimmer can attain 5000-6000ml of vital lung capacity.

Disease Prevention 预防疾病:

The general temperature swimming pool water is often between 26C-28C. Heat dissipation is faster and the energy consumption is greater when the body is soaked in water. The necessity to supplement the heat of the body as soon as possible for temperature balance causes the nervous system to rapidly respond. This again cause human metabolism rate to spend up and enhance the body's ability to adapt to the outside world, and against the cold condition. Regular swimmers of indoor swimming pool are not easy to get cold due to improved thermoregulatory function. Furthermore, the body's endocrine function can be improved too so that the increase in secretory function will enhance resistance and immunity to disease.

Myocardial Function Enhancement 增强心肌功能:

The various organs of human's body are involved when we carry out movement in the water, more energy consumption and increase of acceleration of the blood circulation in order to supply more nutrients to moving organs. The accelerated blood velocity will increase the load on the heart and hence to make it beats more frequently to allow strong and powerful contraction of heart. Cardiac function of a regular swimmer is excellent that the average heart rate of normal person is 70-88 beats/ min, stroke volume is 60-80 ml per beat. Heart rate of a regular swimmer can reach up to 50 -55 beats /min. The heart rate of a lot of good swimmers can achieve up to 38 -46 beats/ min, and their stroke volume can reach as high as 90-120 ml. The role of water in the swimming process is to allow blood contained in limbs to easily return to the heart, and to speed up the heart rate. Long-term swimming will increase cardiac motion and boost strong contraction with conspicuous effects. The thickness of vascular wall increases as well as the flexibility to increase stroke volume of blood. As a conclusion, swimming trains heart to be strong.

The Effects On Human Skin 对人体皮肤的作用:

In the swimming process, in order to ensure a sufficient temperature due to the stimulation of the water temperature, blood vessels underneath the skin participate in regulation function which is important. The stimulus of cold water will cause the skin vasoconstriction to prevent heat spread to the outside of body. At the same time, the body hasten to produce heats, so that the blood vessels dilate and improve the function of blood supply to the blood vessels of skin, so that this kind of long-term and persistent exercise can help to strengthen the blood circulation of the skin.
In addition, the water is a kind of lithe and soft-moving liquid. Due the water wave which constantly causes friction on human skin, our skin is able to relax and rest. That is the reason of fair and smooth skin of regular swimmer.

Weight Loss 减肥:

The swimmers directly immerse their body into water. Despite large water resistance, the thermal conductivity of water is also excellent, plus fast speed of heat dissipation, thus causes more calories consumption, just like a freshly cooked egg cools down in cold water than in the air. The experiments show that: Calories burned in 20 minutes of jogging in standard swimming pool is equivalent to the same speed of that on the ground. The calories consumption in water of 14C is 100 kcal per minute which is equivalent to the heat dissipation in the air of the same temperature in 1 hour. Also, lose weight by swimming avoids sports injury of the lower limbs and waist. Carrying out exercises on land to lose weight will cause the body of obese people (especially in the lower limbs and waist) to bear a large gravity load, hence reduces exercise capacity, cause fatigue, so that the interest of carrying out slimming exercise greatly reduces, and damages joints and bones of lower limbs. On the other hand, a considerable amount of the weight of obese people is supported by buoyancy of the water while swimming to reduce the pressure of the lower limbs and waist and the risk of joint and bone damage is greatly reduced. It is obvious that swimming becomes one of the most effective way to keep fit among all aerobics to allow the people who intends to lose weight to get better result with less effort.

Effects on Cardiovascular System 对心血管的作用:

Swimming plays a significant role in improving cardiovascular system. Cold water stimulation can promote blood circulation by heat regulation and metabolic activity; In addition, water pressure and water resistance have special effects on heart and blood circulation. While swimming on the water surface, the body has to withstand water pressure 0.02-0.05kg/cm2. The pressure increases with depth of diving and changes of physical conditions. The pressure capacity will further increases with swimming speed and the atrial and ventricular muscle tissue will be strengthened. Besides that, the capacity of the heart chambers will gradually increase and the number of heart beat will reduce, so that the heart activities will be reduced to benefit the entire blood circulatory system. Diastolic blood pressure increases in stationary state, at the same time systolic blood pressure decreases, blood pressure value becomes more favourable; the elasticity of the blood vessel also increase as well. According to statistics by relevant experts, the average number of heartbeat of a person in stationary state is about 66-72 times, stroke volume is approximately 60-80 ml per beat. While for the people who engage in long term swimming training sessions, their hearts simply shrink about 50 times in the same situation, stroke volume is 90-120ml per heartbeat.

Protection Of Life 生命的保障:

As we al now, we live in a sphere with three-quarters of waters. Inevitably, we have to deal with water in daily life. That is the reason that swimming is not only a kind of sports, but more importantly it is valuable tools and skills in life. It takes part in many domains of our life: such as water resources development, scientific investigation, flood control and rescue, first aid salvage, etc. There must be proficient swimming skills as backup in order to ensure life protection.