Until research was conducted in the 90's for the Atlanta Olympics, there were many different thoughts on how to cool a horse. It was commonly thought that putting cold water on a horse would cramp the back. Researchers found this to be untrue. The water actually cools the muscles. Very cold ice water put over a hot horse is almost the horse's temperature when scraped off twenty seconds later. The large muscle masses can get up to 107 degrees, and if they aren't cooled rapidly they won't be able to return to normal cellular function. In essence, the heat starts cooking the protein, which causes stiff muscles.
This type of cooling is best when your horse is quite hot: sweating and blowing. You can test with a rectal thermometer. If the temp is lower than 103, plain hose water is good enough and your horse should cool off fairly quickly. However, you don't have to worry about cooling your horse down if he's cooler than 101, he will cool on his own.
If your horse is quite hot, over 103 degrees, put the coldest water you can get on him—even ice water---and then scrape the water off. Walk the horse around for a minute and repeat until the horse cools to 101 or 102 degrees.