In modern hydrotherapy the most elementary treatments are cold, hot and alternating hot and cold baths. Changing water temperatures induces reactions within the body. A cold bath, for a few minutes and at a temperature not less than 60 degree F, has a tonic effect. The small blood vessels in the skin contract during immersion and open up afterward, causing a noticeably warm feeling as the vessels dilate. Hot baths work in opposite way, initially attracting the blood to the skin and stimulating the sweat glands, after which the body cools.
Alternate stimulation by hot and 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 that blood vessels and inflammation of the tissues. With this technique, hot sessions of a couple of minutes are interspersed with 30 second cold dips.

If you are not being treated for any of the conditions above, the alternating of hot and cold temperatures can be very therapeutic. In fact, it is one of the most common ways hydrotherapy is used. (Again, check with your doctor before engaging in such activity).
As mentioned above, using hot and then cold temperatures makes the blood vessels open up, contract, and then open up again. This causes a pumping action in your heart and blood vessels that greatly increases circulation, which, again, aids in healing. This is often a home remedy that is recommended for people with migraines. They take a hot shower, followed immediately by a cool one which suddenly shrinks the swollen blood vessels in the head which are causing the migraine.
A neutral bath, with water at almost exactly body temperature (98degree F), is useful in treating tension, insomnia, and irritating skin conditions. The addition of special herbs, such as elderflower, horsetail, and peppermint, to the water enhances the effect of neutral baths, while the addition of Epsom salts encourages profuse sweating and is recommended for aching muscles. A warm shower should always follow this treatment.
Some other hydrotherapy home remedies are a warm bath for hemorrhoids; hot foot soak for sinus relief; a 20 minute dip in cold water for joint of hands or feet to reduce swelling; a lukewarm body soak to relieve tension and stress; a half hour soak in a warm bath immediately before bed for a deep sleep.
And although we've mainly discussed the reactions of our bodies to the touch and temperature of water, let's not forget about the sound. The sound of water splashing, the roar of the ocean, raindrops on the window pane are all welcome sounds that soothe tension and promote mental relaxation. Many mothers find that crying babies are soothed when they turn on the water faucet and cradle the baby nearby where they can hear it. On another note, babies also respond well to a warm bath when they are cranky.
The benefits of hydrotherapy are numerous and not all are covered here. You may even find that water treats specific ailments for you that are not mentioned. Using some of the home remedy tips listed above, and responsible use of the knowledge of the benefits OR ill effects of water temperatures on the body, we can enjoy the therapeutic effects of nature's most abundant resource.