The wide assortment of external and internal hydrotherapies is central to the ‘nature cures’ offered in many modern health farms and spas. These therapies are based on the belief that a multitude of disorders, from chronic phlegm to cystitis, can be prevented and treated through deep cleansing of the body. Today it is found to be very useful in treating muscular and joint injuries, arthritis and more important relieving tiredness and fatigue.
The practice of hydrotherapy dates back at least as far as the time of the Roman Empire and has been credited with curing everything from hangovers to insanity. Because it is possible to make fine adjustments to both temperature and pressure of water, it is a highly adaptable treatment medium. Some hydrotherapy involves water in motion , either gently bubbled with air, or, as in underwater massage, pumped powerfully by water jets.
The benefits of hydrotherapy are a result of the principles of buoyancy, hydrostatic pressure and water temperature.

1.  The principle of buoyancy of water comes into play when treating muscular or joint injuries. Being in water reduces body weight by up to 90%. Physically this relieves the pressure of weight on our joints and muscles, and, the feeling of weightlessness proves to be very relaxing mentally as well as physically. The weightless effect of water is also recommended for arthritis sufferers, as it eases joint stiffness and improves the mobility of the joints.
2. Hydrostatic pressure offers a form of massage that is soothing as well as healing. Studies at trauma and burn centers have shown that the massaging currents of water gently soothe touchy nerve endings while cleansing the wounds. This massaging action, combined with weightlessness, relaxes tight muscles. It also releases natural pain killers, called endorphins, into our system.
It is important to note that a person doesn't necessarily need to be in a whirlpool to enjoy the benefits of hydrotherapy. A large bathtub at home will do fine, as long as one can relax, stretch out and be mostly submerged, depending on the effect you are looking for.
3. Water temperature has a great deal to do with hydrotherapy. Hot water raises your body temperature and causes your blood vessels to dilate. This increases circulation and greater circulation can speed up the body's healing processes. Some people believe that a hot soak (103 degrees) can help you fight a cold or flu. This high temperature increases the number of white blood cells, which help to fight infection.
As great as a hot soak sounds, there are words of caution to remember. Doctors recommend a temperature limit of 104 degrees and a time limit of 15 minutes. Do not take a hot soak after a workout. Hot soaks are not recommended for people who are being treated for circulation problems, high blood pressure, heart disease, diabetes or pregnancy. (If you suspect you have any of these conditions, see a doctor first).
 Always cool down after taking a hot bath. One recommended way is to splash your body with lukewarm water. Alternate stimulation by hot or cold water causes the blood vessels to dilate and contract; this has a pumping effect on the blood that helps to reduce both congestion within the blood vessels and inflammation of the tissues. With this technique, hot sessions of a couple of minutes are interspersed with 30second cold dips. Remember, hot water causes blood vessels to dilate and cold water causes them to constrict, so cool down slowly so you don't create a strain on your heart.